High error rate/speed issue

[#1]

Greetings,
Since approximately the 4th of November 2013, the connection here has been affected by a high error rate and heavily reduced downstream throughput.
Prior to that date, the SNR Margin was approximately 6.0dB, the downstream sync rate was around 12500, Interleaving was also turned off. The actual throughput received was around about 10.0MB/sec.
After that date, although the SNR Margin and downstream sync rate remained unchanged, the actual throughput received dropped to around 0.5MB/sec - 2.5MB/sec.
In order for a connection with a reasonable throughput to be established, the SNR Margin needed to be raised to 9.0dB and eventually 12.0dB when the previous value seemed to be just on the edge. Unfortunately, there is a hit on the downstream sync rate which doesn't make that desirable. Additionally, Interleaving is also turned off.
Since that time, there have been a number of engineer visits (phone and broadband type), the pair I'm on has been changed atleast 3 times so far. The internal cabling was changed 3 times as well until a complete line switch was performed in order for the cable to be run directly to the building (to eliminate any cabling inbetween). Unfortunately, even after the cabling changes and eventual line switch was performed, the error rate has remained consistently high, and the downstream throughput has mostly been very low.
The router/modem has been changed atleast twice so far without any noticable change. The telephone and modem cables were also replaced to rule them out as well.
Unfortunately, there was no change.
Today another broadband engineer came over, however the high rate of CRC errors only surfaced while the boradband equipment here was plugged in. After the pair on the D-side was changed over again, the router was moved closer to the master socket and the throughput drastically improved for a time, I was able to achieve about 9.52MB/sec (approximately) according to the various speed checkers.
After a couple of hours passed, the throughput went back to its degraded state again, the SNR Margin appears to have "stabilised" at about 4.8dB. At this time, the errors continue to accumulate at approximately 50 per second, the actual throughput I can currently achieve can be as high as 4.50MB/sec, or as low as 0.09MB/sec.
So far, a ton of redundant cabling has been pulled out and unnecessary equipment turned off to see what happens, unfortunately that hasn't made any difference.
I am now left completely stumped as to what could be causing the degraded throughput here.
The official router/modem from PlusNET should be arriving within a few days to atleast either rule out or confirm an equipment fault on this end.
The router/modems that have been used so far:
LinkSys AG241v2
BiPaC 7800N
BiPaC 7800VDOX (This one is about a month old at this point)
The SNR settings on the routers themselves are at their default rate.
Current number of errors after the connection has been up for just over 3 hours:
SF (CRC) Errors:  656231
HEC:              1424084
Errored Seconds:  10542
Severe ES:        10542
Noise Margin:     4.5   dB
Connection Rate:  12,815  Kbps
Line Attenuation: 25.0  dB

[#2]

Hi there and thanks for the post.
I'm sorry if it seems that we've been through a lot of rigmorole on this one, unfortunately some faults mean a lot of things have to be ruled out as part of the investigation.
Please keep us posted and let us know how things go with the new router we've sent out. I don't mind intervening after that if that would help things along.

[#3]

Greetings,
At this point, it's just the bizarre nature of the fault that has me confused.
I've now spent the last 24 hours (or so) re-doing the cabling here and turning off/removing unnecessary equipment without any noticable effect.
The only detail of note so far: The SNR Margin jumped to about 9.8dB (without a resync) some minutes ago, the downstream throughput is now stable for the time being.
I don't know what caused that to occur, or for how long it'll last.
That still leads me to believe that the fault is external (though it seems to be next to impossible to narrow it down, aside from an aging infrastructure),
The PlusNET bundled router/modem should be arriving in a few days. I'll admit that I'll be slightly disappointed if it does turn out to be an equipment fault. But if it solves the problem.. oh well.

[#4]

Greetings,
The new router/modem arrived yesterday.
So far so good. The throughput issues I was experiencing with both BiPaC routers has now gone.
The modem cable that was supplied with the new router is also in-use, it's a much better grade than the one that was previously plugged in.
The downstream sync rate being received on the new router is much higher than what I've previously been able to attain with an SNRM of around 6, it is currently sitting at 14495.
The actual throughput is about 11.93-12.32 Mbps.
I can only conclude that the previous routers were either faulty, or although workable in some cases, not entirely suitable. The only mystery that remains is why the 7800N performed without a problem until the end of october/beginning of november 2013.
While the connection is stable with a high sync rate, it's probably best to not experiment any further.
Edit: The Downstream SNR Margin dropped to 4.4-4.5 without a resync occurring. After that occurred, the downstream throughput I was receiving dropped through the floor. I am currently just about averaging 1.50Mbps down. The error rate skyrocketed.
Although the Technicolor TG582N is far more tolerant, it seems that the latest SNRM reduction due to stabilisation went right passed it.

[#5]

It apparantly goes on, and continues to become far more frustrating.
Another engineer came today, the line came back all clear, and an RF3 filter was fitted as a prerequisite for any REIN escalation.
A request to for the cable to be re-terminated in the Junction Footway and Distribution Point fell onto deaf ears, as well as the possibility of being moved onto a better (relative) pair in the cabinet. That seems to be very hit and miss depending on the engineer that comes arrive.
At this time, the SNR Margin is sitting at 12dB, the sync rate is 9015 kbps. The actual speed being received is quite consistent.
My main problem at this point is the fact that an SNR Margin as high as 12dB is even required, whereas one of atleast 6dB worked prior to the 4th of November 2013.
If this is a REIN fault, it started about a week after we had very strong gale force winds here. There were no christmas lights hanging at the time, no other visible changes either.
During the 24-26 hour window (when the Technicolor TG582n router arrived and was plugged in) from Saturday to late Sunday night, I had an SNR Margin of 6.0dB as well as a sync of 14495 (aside from an earlier sync of 14990, that is the highest I have ever managed to achieve), and could achieve a maximum downstream throughput of 12.32Mbps. There were very few errors. At around 21:57, the SNR Margin dropped to 4.5, the errors sky-rocketed and the downstream throughput dropped like a stone to around 1.5 - 2.5 Mbps. Even after a resync occurred on the Monday with an SNR Margin of 6.0dB, the error rate was still ridiculously high, and I could not achieve the previous high downstream throughput.
Additionally: Whether related or not, a line switch was performed back in February 2014. Although a new cable was run directly to the building that contains the master socket. The original dropwire was left in-place. I cannot confirm either way whether that is contributing to the problem or not. Would there be any way of getting it removed completely. If it were to be removed, the cable-clips would have to be lifted off and the cable pulled through a hole near the window frame.

[#6]

Hi AF-Domains,
I've had a look today and can see that our Faults team have raised an escalation with our suppliers and they'll continue to monitor this on Ticket: 81478171 for you. i'm sorry to see it's not been resolved for you yet. Hopefully this escalation will be dealt with and the issue resolved for you soon.

[#7]

Greetings,
With any luck it will help in this matter.
By some bad (or good?) fortune, there was a further bout of instability at around 18:00 yesterday which resulted in the SNR Margin being increased to 15dB automatically.
Something is continuing to degrade, though who knows what the ultimate cause(s) will end up being. No doubt it'll be hideously anticlimactic if/when it is tracked down.