Disconnections and error problems

[#1]

I have a fault logged with Plusnet at the moment because my (brand new) router - a Fritz!box WLAN 7170 was losing sync and disconnecting. I use this router because it has built-in VOIP.
(Plusnet - if you are reading this the question ID is #37059614)
These problems started following the upgrade to ADSL2+ - before that our connection was always stable.
Plusnet says that only the Thompson Gateway is guaranteed to work, and have sent me a TG585 v8 (firmware 8.2.7.7) which I have plugged into the BT test socket with the filter provided. The only other equipment on the line is a standard BT cordless phone connected to the same filter.
Since using this there has been  no loss of sync but one reported loss of internet connection in a week:
Warning Dec 5 23:00:16 PPP link down (Internet) [87.112.191.2]
However the router is reporting very large numbers of errors in particular HEC errors, an upstream SNR margin of 27.5m and the downstream margin has gone up to 9dB (see below).
I am about 700m from the exchange so line should be very good.
Advice please on:
- whether Plusnet are right about the router (is there any reason why the Fritz!box should not work - the manufacturer claims it should work fine with ADSL2+)?
- are the errors evidence of a problem with the BT line?
Uptime: 0 days, 23:14:00
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 888 / 19,432
Bandwidth (Up/Down) [kbps/kbps]: 440 / 17,866
Data Transferred (Sent/Received) [MB/MB]: 178.87 / 702.83
Output Power (Up/Down) [dBm]: 11.0 / 20.0
Line Attenuation (Up/Down) [dB]: 9.0 / 19.0
SN Margin (Up/Down) [dB]: 27.5 / 9.0
Vendor ID (Local/Remote): TMMB / IFTN
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 0 / 0
FEC Errors (Up/Down): 36,032 / 107,724
CRC Errors (Up/Down): 1,281 / 119
HEC Errors (Up/Down): 1,368,653,824 / 1,994

[#2]

HI GuyMoquet,
Although there seems a lot, it's showing errors that your router has reported have been corrected, which is exactly what it's supposed to do. Are you suffering any poor performance from your connection at this moment in time?
Jojo

[#3]

Thanks Jo,
The connection is at the moment stable, apart from the one disconnection I mentioned. The SNRM in particular upstream  is higher than it should be according to other posts on this forum - should be more like 6dB, so wondering whether this is evidence of some noise on the line.
The Thomson router isn't really ideal for us for two reasons:
- it seems not to work well with the Solwise homeplugs we use for internal networking (essential as our BT master socket is not in the same room as our PCs and I am not able to use Wifi for my work laptop. We are finding that the PC keeps losing connection with the router, possibly due to  an IP conflict.
- we need to use VOIP for a second phone line which is why we'd rather use the Fritz!box which is designed for this purpose. 
So I'd like to try to establish why the Fritz!box did not work - and so far it's looking like there is some problem on the line, although the Thomson is managing to maintain a connection better.

[#4]

Hi GuyMoquet,
Don't really know much about the fritzbox, sorry. As for the ip conflict I would just manually assign an ip to one of the computers and that should cure that.
Jojo

[#5]

There is no reason in principle why the Fritzbox shouldn't work, as you say its ADSL2+ compatible. The equipment at the exchange for 21CN is different than the old 20CN and therefore when your upgrade to ADLS2+ happened, the exchange equipemt is most likely now different. There is evidence that some routres work better than other with certain exchange equipment.
From a PlusNet point of view, they can't really be expected to know about all the different routres out there and so the support people are really only able to provide support on the routers they supply.
Regarding some of your q's

Quote

The SNRM in particular upstream  is higher than it should be according to other posts on this forum - should be more like 6dB, so wondering whether this is evidence of some noise on the line.

The upstream SNRM is high because your upstream is capped at 440, and is not indicative of a problem.

Quote

However the router is reporting very large numbers of errors in particular HEC errors, an upstream SNR margin of 27.5m and the downstream margin has gone up to 9dB (see below).

I'd take the Upstream HEC count with a 'pinch of salt' as I suspect that been misreported by the router. The other counts are not unreasonable given the pretty high downstream speed. Initially you had the normal 6db downstream SNR target and I suspect the router was trying too hard to hold a high synch speed and thus getting significant numbers of errors. The exchange has increased the target to 9db which should cause a slight drop in synch speed and thus error rates.
My guess is that the Fritzbox was also trying hard to hold a high synch and thus dropping out with high error rates. Probably the best thing is to let the exchange stabilise the line with the TG585 connected, this should establish a suitable target SNR and synch speed. Once thats happened , check the synch speeds/error counts and then try the Fritzbox again and see how that compares.