I have a D-Link and have had similar issues. Once in a while (used to be once a week on average) the router would suffer a crash in its wireless stack. This can be easily verified: you cannot browse to the router's admin site - it's down. But if you connect a laptop to an ethernet plug on the router, you can surf.
If you look at the D-Link forums for the 655 and 825 models, you'll find dozens of complaints. D-Link ignored this for several months and the started to try addressing it by a series of firmwares. In the case of the 825 they even came out with a B-model that fixes most problems (not that it helped anyone who had the A already).
The latest release seems to do the trick for me: I now average about a month between crashes. What seems to exacerbate the crashes is downloading large quantities of data, particularly if you use BitTorrent. The higher the download speed, the higher the chance of a failure.
So, upgrade to the latest firmware, avoid going over 500Mb/s downloads and you should be fine. Alternatively, do what other people on the forums suggest and ditch the D-Link.