Yet another .02 and sorry to hear of your troubles, i have been going thru similar issues

if interested in my saga you can check it out here

http://forums.att.com/t5/user/viewprofilepage/user-id/2712205

 long story short  if you believe you have tried everything (see Below) your problems could be related to a bad NID

Ran Cat6 cable to the Gateway
Tested service household Power on and off to eliminate RF
Service was poor and unacceptable; UVrealtime reports Bridge-Tap; distance to VRAD 1000’

Called AT&T and whan installeren their installer came out, He found that one of the legs wasn’t completely tied down by original installer
Service stable; UVrealtime reports Bridge-Tap; distance to VRAD 900’

Started working with AT&T Tier 2
Moved RG to NID and connected to the Diag port and tested - No bridge taps

Conclusion - NID is causing Bridge-Tap

Connected Spare Cable directly to NID Diag port
Crimped a RJ11c4 on a spare cat6 cable and connected to the NID and RG

Service Stable; Bridge-Tap Gone!; distance to VRAD 800’