FTTC noise margin drops over the course of several days

The downstream noise margin will start off high on a new sync then over the course of several days will drop until the connection is no longer stable and it resyncs. Can this be a type of fault? Openreach say the line tests ok for it's 700 meter length but even so it doesn't seem too stable and it ends up running slower than it should. I recently had a DLM reset and I'm syncing around 37mbps down and 5.5mbps up but I expect the downstream to come down to the high twenties over time.

I'm just wondering what is going on with the constantly slowly dropping noise margin, the line is never stable for more than a few hundred hours at most. It doesn't drop at a certain time, just a slow decay until the connection drops but as soon at it resyncs it can be as high as 10db or more before slowly decaying once again.

Current stats taken from my Sky ER115 router
Broadband Link Downstream Upstream
Connection Speed (Kbps) 37494 5542
Line Attenuation (dB) DS1:21.1 DS2:58.5 DS3:89.4 US0:4.2 US1:47.8 US2:62.6
Noise Margin (dB) DS1:2.9 DS2:2.9 DS3:0.0 US0:6.0 US1:6.0 US2:0.0

Any ideas?

Getting a regular stream of FEC errors. ?

Tried a different router ?

Describe your set up, not got extension wiring connected have you ?

In reply to a post by Zarjaz:

Getting a regular stream of FEC errors. ?

Tried a different router ?

Describe your set up, not got extension wiring connected have you ?

Sky doesn't show any more stats than what I already posted and as far as I can tell only Sky routers work on Sky lines unless I'm willing to shell out for the select few which are confirmed to work. The router is connected to the master FTTC socket and the openreach engineer said it was ok.