ATT DSL dropping signal on two different lines for one location

We have a really wierd issue going on at a client location that involves a DSL connection that has pretty much boggled our minds here. Here is a recap:

Our original DSL line was having issues where it would drop connection - not entirely, as the lights would still be on, but there would be a sort of freeze on the line where no web pages could be pulled or the server connected to from across the net. A quick reset of the modem or waiting 5-7 mins would fix this.

We thought that it could have been viruses, trojans, or some poorly capped lines, or even licensing issues on our 2003 server, but all those areas were tackled and we still had the issue. After much haggling with ATT, they came out and installed one brand new line for DSL use only. This line DOES NOT have any phones or fax connected to it - it is strictly for internet usage.

They gave us a new modem to use, along with a new PPPoE logon name and pass, but we kept the old modem in use (same models both old and new) due to some firewall settings we couldnt change on the new one. Nonetheless, the new line still experiences this drop! There is a neighboring office in the complex that has DSL from ATT but it works just fine.

We have narrowed this connection drop to an ATT issue as nothing inside the network is having issues. Connections to the server within the LAN stays up and running when the modem signal goes down. Just so you know, this location uses a static IP for their DSL. THe modem used is the newest one from ATT, the 2WIRE 4 port wireless gateway, the HG series. This office is on their THIRD Modem already, so we highly doubt its a bad modem we are dealing with.

We are at a point where suggesting Comcast cable internet is the only solution, as anything mixing internet and phone lines at this office is becoming taboo. Can anyone recommend any other areas to check before we call it quits on this 2 month ordeal with ATT DSL?