I just got a DIR-615 plugged in yesterday, and have experienced pretty much the same thing as you, although only once so far. I had it connected and working, had customized some settings (very few), had internet.
I left for awhile, during which time my desktop computer went to sleep. When I returned, I had no internet and couldn't access the router. I can't remember whether Win XP said I had "limited connectivity" or "network cable unplugged". Anyway I had nothing.
I rebooted the computer (nothing), then powered cycled the router (still nothing). Only a hard reset brought it back.
Earlier in the day, I encountered a time when the router "forgot" the new IP address I had given it (and saved the setting).
I too am using HW C1, FW 3.01
I hope there isn't a bad run of these!