I definatly know where you're coming from, exactly the same thing happens to me. Is the router far away from the PC, so it could just be on the brink of connectivity when it drops?
I asked this question a few months ago, got this as a reply:
(Note: This isn't the exact thing you need, just an example of it)
http://www.homeplugs.co.uk/
Plug, install, go.
Would have bought some myself if I had the cash.