I've been lurking this forum for nearly a year now, since getting a DIR-655, and I've had random loss of wireless access at least once a day, sometimes 2-3 times a day.
While browsing around the settings looking for a way to increase the DHCP lease times I noticed the expiration times were way in the past. Like January 1970 or some such.
Then I looked around and saw that date/time for the router was wrong. ("Current Router Time").
So i set up Enable NTP Server (Tools->Time->AUTOMATIC TIME CONFIGURATION).
Ever since doing that I haven't had a single loss of wireless connectivity. Rock solid. For weeks now.
For those of you who are experiencing dropped wireless connections, this is one more thing for you to try. I hope it helps you.