I doubt the problem is native to WoW since WoW generally isn't going to make significant configuration changes to your computer's network interfaces and related systems. Does it seem like you're having ANY network trouble on your computer at twenty minute intervals, or is it just when WoW starts up (and has been running)? If you are playing on a Linux system (and props if you are), let us know what the ifocnfig output looks like once you have trouble (this also applies if you're playing on a Mac). If you're playing on a Windows box, would you post the result of ipconfig (a command in the command line) so that we may check your general network interface configuration?

Also, when this happens again, would you mind trying to ping some website? It would be interesting to see just what sort of network abilities your machine is left with after this sort of failure.

Sorry I couldn't be more help right off the bat.