Line speed increased but download speed stuck

[#1]

Hi all,
I’ve been running a Billion BIPAC 7800n since early March 2012 and from the minute I got it installed, successfully tweaked the SNR settings to raise my download speeds from around 550kB/s to a much better 765kB/s with a downstream line speed of 7,616 kbps which it consistently maintained  :).  Then about a month ago, there was a slight drop in line speed to about 7,585kbps followed by another drop a few days later.  Suspecting a router problem, I rebooted it, which unfortunately lost the connection altogether – the DSL light didn’t even light up, so looked like a fault in the modem section.
Anyway, after doing all the normal diagnostic checks (plugged the router into the test socket behind my Solwise faceplate, replaced the line connector etc. without success, eventually tried an ancient Belkin router, which immediately connected but with a much slower download speed of around 365kB/s (and obviously not tweakable!) 
So I bit the bullet, bought and installed another Billion BIPAC 7800n and started the tweaking process over again.  However, although this once again resulted in a lift in the downstream line speed (up to 7,008 kbps so far), the download speed remains at the paltry 365kB/s level and has continued to do so since the new installation 25 days ago. 
Can somebody tell me - has my line been capped for some reason?
Brian G

[#2]

what does this say https://portal.plus.net/my.html?action=data_transfer_speed

[#3]

Yes, understand all of that, apart from the ‘Current line speed’ figure.
Maybe I should try to illustrate my point visually.  I've attached a .pdf with a couple of screen clips.  The top clip is from the Plusnet Speed Checker; the speeds on May 27th are those I’d been consistently getting for months until my apparent router failure.  The speed measured yesterday is what I’m been getting since replacing the faulty router 26 days ago.
The second clip shows my 7800n’s reading of the current line stats; the downstream SNR figure was around 15dB with a downstream speed of around 4200 when the router was first installed.  The figures above are those I got after tweaking the SNR settings once the recommended 10 day waiting period was up.  But the ‘Download Speed’ of 2913 kbps (364kB/s) recorded by the Plusnet speed tester hasn’t altered one iota in the whole 26 days and that’s why I’m mystified and pee’d off. 
Brian G.

[#4]

Hi xbeemer,
Sorry to see that you seen a drop in the speeds since the router problems. I've just run through a few tests on your line and I can't see anything that would be banding or restricting your line in any way at the moment however I can see the sync speeds sitting at around 7003kbps.
have you had the router connected at the test socket behind the faceplate since you purchased the new one to see if this helps with the speeds?

[#5]

Hi Brian,
I suspect you have shot your self in the foot by messing with the line settings.  Leave them alone and let the DLM manage the line so that the true state if the line can be "seen".  If the DLM wants to set the line at (say) 12dB then usually there is a good reason for doing so, which until addressed, messing with line settings is completely futile.
Remember that DATA speed measurements are the effective speed of the line AFTER end to end data retransmission and other error corrections.  Artificially squeezing more synch speed by fiddling the SNRM can cause a significant rise in error rates, thereby significantly degrading the usable data rate.
Can you please post a full set of stats with the router running unadjusted settings so we (and PlusNET) can see the true characteristics of your line.
Kevin