Interesting P2P issue

So here's the deal. My friend (who lives in Canada) has Shaw internet, which is a high speed cable connection. Him and his roommate have multiple computers on the same network split through a router.

Everything works fine except when one of them does any amount of P2P networking (Bittorrent, uTorrent), the whole connection starts having severe lag/desync with Minecraft and Anarchy online servers. The issue seems limited to these (have not tried any other MMOs) and games like Modern warfare (2 and 3) both work perfectly. All other internet activity as far as we can tell is unaffected, only those two programs.

We really need some help here ><

We've tried 3 different routers, 2 different modems (they have moved since and got a new modem from the same company.)
Shaw said there was nothing they could do about it if everything else was working fine.

I'm thinking something to do with either ports or Dynamic IP issues, but I may be way off the mark. I don't know a whole lot about networking.

Any advice would be great!

Have they properly setup their torrent client ?? --- If left set to unlimited Upload speed the torrent programs will saturate the upload speed causing there to not be any headroom for communications between other servers - not only causing the P2P download speed to suffer but also the rest of the internet connection to have problems.

Make sure they are setting up the client to only allow about 70% of the upload bandwidth to be used and see if that helps