I used to have a D-Link router and had no end of trouble with it. It could go for weeks without fault, but then be thoroughly unreliable and the only solution each time was a power on reset. I did all the usual flashing of firmware and tried several updates issued by D-Link specifically to resolve my problem. The support was ok, but fell apart when they failed to rectify the problem. Searching web forums revealed that I was not alone. I'd recomennd googling!
The retailer who supplied the unit finally admitted that there was an inherent problem with this model and offered a replacement from a different manufacturer (took 9 months, but i persisted).
I now use two U.S. Robotics routers and both work fine.
If you don't want to shell out for a new router, you could buy one of those wireless remote control plugs to P.O.R. your duff router.