Can't connect to servers on 2 different pcs

Hi all,
Just made a new PC and stumbled across a problem. I can join game servers with no problem, however, as soon as another computer on the network also joins a server, same or different, I'll immediately get disconnected during their connection stage. I can't rejoin the server and the only way to get back to being able to join servers while no other computers are playing is to unplug the main internet socket. For some reason, unplugging the ethernet cable or resetting the router won't cure this.

Any ideas?

These games are using Steam to access, specifically Team Fortress 2, Counter Strike Source, Garrys Mod etc etc Any Steam game connection will cause the problem.

This problem didn't happen at all on my previous PC, we could use all three computers at the same time on the same or different servers.

It's a home built PC, Windows 7, no manufacture software installed at all. Tried all the simple stuff such as firewalls etc Mac addresses are different.

thanks

(Also, yes I have tried asking on Steams own forum but of course, can't expect much there. Also sent in a ticket but expecting a copy paste reply at best.)