Covad / covad.net → Change in noise margins

I've had a problem develop over the last couple of months where my up stream noise margins have gone South. My downstream are perfectly fine and my download speed is never a problem. I have the 3.0/768 service and I've been with COVAD for a very long time with minimal problems. However, my up stream keeps dropping to about 225 and my noise margins go down to 300s. I'm calling to have it re-provisioned every couple of days, maybe a week and then it goes to 450 or so, which is still pretty bad. But at least I'm getting low 700s on the up stream again. It's an issue because I have a personal FTP/File server at home that I access from work.

This wasn't a problem before, but has become one in the last couple of months and I'm not sure where to look or if there are any possible solutions? I've now removed all devices (although I haven't added any before) except my primary line and one line for our TiVO unit which dials out in the middle of the night. I've changed the filters on the two lines that are plugged into the jacks. COVAD has run line tests were look OK as far as problems go.

Does anyone have any suggestions? I'm sure calling Verizon is pointless, particularly since my downstream is perfectly fine. Would it be worth the time to have them come and check the wiring in my basement? I realize this won't be "cheap" but I don't mind spending a little if I thought it might solve the problem.

Thanks in advance for any ideas or advice.

So... any ideas where I should be looking for the problem at?