I also have a AT&T supplied 2Wire 2701HG-B gateway with built in wireless that has worked great for years, until approx a week ago... Recently, the gateway has started "rebooting" for no apparent reason. Google search finds a lot of similar issues dating way back, but few are clear on a true fix. Appears those who have contacted AT&T support have been given the runaround with canned flip chart answers from foreign sounding individuals... always pointing the problem back to the user... typical... And now I find this thread that has been going on for a while now.

So far I have:
Cleaned all my connections.

Tried it with two different phone cable runs to my outdoor phone box (one a direct tie with Cat6a cable).

Disconnected my 2.4GHz phone which was sitting close to the modem (BTW, the modem worked fine for years with the same phone sitting there but I moved it, just in case...)

Installed a phone line filter on the main phone line feeding all my house phones. Previously, I only had the filters installed locally on some of the phones in the house, but it worked fine. 

Tried it with no house phones connected to the phone line at all.

Tired different Ethernet cables from the main PC to the modem.

Eliminated all Ethernet connections except the main computer, including any wireless linkups (I have had up to 2 Ethernet computers linked, 1 XBox Ethernet link, 2 smart TV wireless links and 1 laptop wireless link up and running at basically the same time in the past with no issues, that is before this "rebooting" issue randomly started).

Still same results, it just reboots for no reason, anytime of the day, sometimes multiple reboots at a time. It cycled for almost a full day last week, then reconnected like nothing happened.

Obviously this is very frustrating. Ideas?

Thanks in advance