My setup is a dsl modem, and a dlink di 524M router. I'm also using a Witopia VPN which runs through OpenVPN.
I've been having trouble with the internet connection dropping very frequently. It comes back shortly, without even a router/modem/computer restart. This happens as frequently as every ten minutes. Occasionally (not often) it will last as long as an hour or two without dropping. When it drops, I can get it back almost immediately by clicking Reconnect in the OpenVPN GUI and letting that do it's thing. It's worth noting that I'm in China. Calling support is a bit difficult because of that. Also I don't really understand all of the router's software, although I've got it generally figured out.
I've tried a bunch of stuff, attempts to diagnose and/or fix the problem. No success with any of the following:
I've power cycled both the modem and the router.
I've tried an ethernet connection to the router.
I've connected without the VPN.
I've disabled IEEE authentication on all connections.
I've checked for viruses.
I've tried lifting it off the ground so as to prevent overheating.