I had a sudden onset of this problem with this model- after a while I realised it tended to cut during the evening after intensive use and felt the router. Suffice it to say it was uncomfortably hot. Tried to improve circulation in any way I could, upgraded firmware and raised a ticket with D-Link who suggested I used ethernet instead. After that I took it back and traded it in under warranty for a Netgear DG834PN router which I couldn't be happier with.
The key thing is to establish if it's a) wifi or b) adsl that's dropping. The easiest way to do that is to type 192.168.0.1 into your address bar next time the internet drops. If you can get the D-Link web interface then it's probably the internet, and logging in will confirm that on the status page. If you can't get the web interface wirelessly, plug a laptop into the router via ethernet and see if you can get the internet that way.