I wanted to add that I was having this problem today. It just started a few hours ago so I googled, found the threads on here, and tried things out.
On my DIR-655, first thing I did was try upgrading from 1.21 to 1.31 but the problem persisted. When I read this thread, I realized that just when the problem started showing up, I updated Symantec Endpoint Protection on a wired machine, including the firewall features, and after rebooting it, my wireless computers (3 in all) started dropping like flies. "Wireless Restart" showed up in the router log every minute or two, like clockwork.
I just shut down that wired machine and... whaddyaknow, the wifi status has been rock solid again, like it has been for the 2 past years I've had my DIR-655.
Now, I'm wondering if the firewall feature of SEP is doing something strange... I didn't have that part of SEP installed before, but upgrading to the latest release, I figured I'd include it and disabled the Windows 7 RC firewall instead. I guess I better rethink that, or at least do more tests and see if indeed it's the SEP firewall causing the DIR to drop wifi sessions like hot potatoes.
So, before you wonder about whether you have a bad router, you might try it out with all wired-connections unplugged just to see if that solves it. Whether wireless restarts are a problem with the NIC on some systems, or whether the DIR is mishandling some data in a very bad way, I'm not sure, but the wired-NIC is definitely the cause in my (and apparently other) cases.