We recently replaced a DIR-825 that worked well for a couple of years, but suddenly died, with a new 855 with firmware version 1.21NA.
Ever since, it's been driving me crazy, I thought it was something on our network, like inbound attacks or what not, and i've been pulling my hairs out trying to figure this out until I came across this topic.
Someone in another thread suggested disabling the DNS relay solves the problem. I just tried this now and will report in a few days if it is successful.
http://forums.dlink.com/index.php?topic=7591.15
Adi