I've been wrestling with this one for years, on and off. It's a problem with my parents' ADSL and since I'm staying with them at the moment I thought I'd take another swing at it. I'd register, but I'm currently afflicted, and it would take me a while!
I'll start with a description: Once in a while, when a phone call is being made/received, there is audible crackling on the line and of course the noise margin (normally 11-13 dB) nosedives until sync is lost. When the modem (a Belkin N1 - would not recommend to a friend) resyncs with the exchange, a really, really low speed is negotiated (sometimes as low as 135Kbps). After this sync, my router will show a massive noise margin, perhaps 35dB. Interestingly, I have never seen this problem cause the upstream bandwidth to drop, is this fixed at the exchange, even on Max, or is it just capped to 448Kbps? Can it go lower?
As soon as I notice this, I reset the modem and it then resyncs at a normal speed and noise margin (typically 2500-3000 Kbps / 11-13dB. Obviously, I am then trapped on a ridiculously low IP profile, and it can take days for the exchange to adjust it upwards again, even if there's a huge gap between profile and current sync speed.
ISP - Nildram aka TalkTalk Business aka Opal (formerly with Zen, until they temporarily priced themselves out of the market, same problem existed)
OS - Various Macs running Leopard or Snow Leopard
Wholesale number checker - Expects 2Mbps on fixed (sounds about right to me knowing what the line is actually capable of 99.9% of the time), or 250Kbps - 1Mbps on Max, but obviously that's based on the problems caused by this anomaly.
Attenuation - very stable at 54dB down / 31dB up
Things I've done to try to resolve this -
-Swapped out all microfilters, more than once, actually.
-Eliminated the possibility of faulty connected equipment - This problem has persisted through about 4 different modems, some wired, some wireless, and 3 different DECT phone systems. All of the equipment has changed over the years, the fault has not.
-Isolated the master socket from the extensions as much as possible - The system is a bizarre combination of star-wired and daisy-chained and the master socket is pre-NTE5 so there's no test socket. There were originally two sockets wired directly to the DP, via a junction box, with a further 4 (four!) extensions daisy-chained from the capacitor-less non-master. Since only the two terminations connected directly to the DP were being used I have eliminated the four chained extensions from the circuit by removing the relevant wires from the crimpers at either end of the extension cables (ie, at the extension socket itself, and at the secondary termination box).
-Isolated master by disconnecting the 2nd termination - Okay, I opened up the box and disconnected everything in order to see if the problem would continue. It did. I don't know if I'm technically allowed to do this, with the demarcation point being rather fuzzy on a star-wired system, but, frankly, all of the Openreach equipment in this house is decades obsolete anyway, so would they really care?
Any thoughts/advice? At this stage I will try anything.