I've read a few instances of this problem, seems to be happening to some people who have updated to the 1.05 frimware. Seems to be rectified by doing one of two things:
1. Go back to the 1.03 firmware (which you can get ont the DLink site)
2. Do a total reset (push the reset button in on the back with a paper clip) then re-installing the 1.05 firmware. Seems going right from the 1.03 to the 1.05 isn't working correctly, you need to start from scratch before updating.
I have this router and have never had a problem with it, so it seems to be a hit or miss thing. Hopefully one of these two options will work for you.