MWEB Guy, maybe as a general rule in the uncapped FAQ or terms of service you guys should add that P2P software such as torrent client / application will effect services such as Steam. I know this as I have some knowledge on how p2p works with all the connections etc they make to the services required for torrents to work. Most average Joe's would not understand the whole concept.
Once your P2P app such as Vuze or uTorrent is open, even if you have no files seeding or downloading, you become part of the peer network, the open connections are basically created. This will at times prevent Steam from even logging in. The minute you exit the application completely (not just to task bar), as in out of memory and quit it, Steam will connect. If you manage to connect with a P2P app running, your ping will jump around all the time, and that type of latency is horrible. On the odd occasion you can play even when downloading a file via P2P, but it will be nowhere near the ping you should get.
Its simple, want good latency in game, close all other network / internet based applications....