Noise Margin issues

[#1]

Moved this out of another thread as I think it got lost (well it didn't get answered!).
My noise margin seems to be all over the place, today I'm connected at 8702kbps (that's the fastest I've ever connected), but the noise margin was instantly 3dB (I have just rebooted). Yesterday I think I rebooted a few times and either got in the low 5000's (when I looked I had an 11dB noise margin) or really fast speeds (between about 7800 and 8200kbps) with a very low noise margin (dropping towards 0dB). I believe the line is set to connect at 6dB, but I don't understand why it connects very fast (and unstable) or (relatively) slow.  I think I've been connected in the 6000's before and had a really stable connection.
It's quite frustrating as I keep needing to reboot the router if the noise margin drops too low. It'd just be nice to have a target SNR of around 7 or 8 dB (unless it needs to be in 3's then 9dB would probably be better) and a stable, but reasonably fast connecting rather than either of the extremes. The current line speed is 4500 (obviously due to one of the low connect speeds) so that proves I've got a pretty strange line. I've done all the usual checks, used the master socket and checked for interference (there is a bit from the TV but nothing that explains these erratic line connects). Is there anything else to try? The filter is fine (I've tried with an alternative too).
Currently connected at 5213kbps with a 6dB margin (it's just lost connection having been connected at something like 6300 with a 6dB noise margin).

[#2]

I take it your on the 21cn going from your sync's ?  I dont know much about that ! Have you tried a different router?

[#3]

Yeah I'm on ADSL2+ now (I believe), haven't tried a different router, but I see no issues with it. I've previously tested things all over the house to eliminate interference and had been quite happy for a month or so. Now it just seems I have this really fast or relatively slow connection speed issue.
I've just now rebooted the router again and connected at 7152kbps - all the same electrical appliances are on, it's just a couple of hours on from 3 reboots that connected at jsut over 5000kbps.

[#4]

I had a similar problem, although not with quite such a variation in noise margin and speeds, and invested in a Billion BiPAC 7800N router. A bit expensive but even though the noise margin fluctuates, and sometimes drops as low as 1.5dB, the router holds sync and the IP profile hasn't dropped since I set up the router. My point is that you might not think the router is at fault, but it would be worth at least trying a different one. Any chance you can borrow one?