I had this exact same problem for a VERY long time.  We had multiple techs out here, line techs, etc.  It turned out to be at the CO.  They had to switch pairs WAY back upstream.  There were no errors on the gateway, and they must have changed everything in the house at least 3 times.  It took me almost 12 months to get it resolved.  FINALLY, it's over, but it took a lot of escalation.  It's not the box, it's not your house, it's not the cable, not the d-marc, not the barrel connectors.  It's somewhere in the line.  Unfortunately you have to get line techs involved who are union workers and VERY difficult to deal with.  It took 4 episodes of them getting working back from the street pedestal to the CO until they finally found it.  Then, they had to swap out significant portions of what I believe to be terminal blocks because they were corroded..

This was a freaking nightmare that took a long long time.  Nobody believes you, they don't take good notes, and Uverse is loathe to escalate past their own support, yet the issue was physical.  Then the line techs are mostly "dial tone" guys who don't appreciate that the hardware has to be of high quality and its not enough to just show continuity.

Just keep escalating.  I found that I got good results when I finally stopped allowing techs to come to my house any more and I called to cancel the service.  ATT apparently allows the customer service people who field service cancellation calls to escalate more effectively.

I hope this helps.