Getting disconnected after 20 secs

I bought 2x GuildWars: one for me and one for my girlfriend. With the 2nd, box, I also bought a computer so she can play the game. My problem is this: whenever she plays, we both get disconnected like after 30-50 seconds of gameplay. Same when shes alone on the network. But this only happens when we play, as I did HUGE multiple downloads without any problems!

So I thought it could be the onboard network card sending crap and flodding... I installed a DFE-538TX (which works well, took it out of my server) and same problem is happening: after couple of seconds, we simply lose internet for a minute or two, not even being able to ping google.com.

Of course, I called the tech guys who just sold me the stuff, none of them could help me out: you changed the cables? you re-installed the drivers? you re-installed windows? you flashed the board? in the end, they suggested that my router was defect and i had to change it. LMAO... Anyways, would any of the techies here have any suggestions? I'm running out of ideas here.

Puter:
AMD 2400+ Sempron
K7S41GX

Router:
Linksys WRT54G

Anyone? *cries*