I noticed a fair amount of posts with a similar problem, and found a reply from jakethecat that is pretty much verbatim with what the ATT tech support said. I thought it would be a good entry to the FAQ. (btw, I have a 2701HG-B, and resetting to factory defaults is what fixed my problem)
said by jakethecat:Three general things will cause a 2Wire to reboot with out permission.
1. The power going into the 2Wire is unstable - corrected by moving it to a different circuit or connecting it through UPS.
2. The 2Wire (or it's power supply) is no longer functioning properly on a physical level - hit up support@2wire.com and see if they will send you a new power supply (or your ISP for a full replacement if you are within warranty).
3. The software running the 2Wire is not in a stable state - reset your 2Wire to factory state by holding the little red button in the back for 30 seconds. If the issue persists, disconnect all computers from the 2Wire. If it still reboots the 2Wire may need replacing.
and 4. Tech support additionally told me to run SpyBot to make sure there wasn't anything on the computer that shouldn't be, trying to open too many sessions.