Sometimes when i'm playing, and other friends are, when I try to join their server i'll just get "awating connection" constantly, yet I can join other random servers straight away and get the "waiting for authentication" message whereas I don't get that when it won't let me connect

Rebooting my computer seemed to fix it or so i'd thought, after about 2 minutes of being ingame i'd just get Connection Interrupted ingame and have to disconnect, once trying to reconnect to the server it'd be the same again, constant awaiting connections

Any ideas? id post 'specs' and stuff if i felt it was needed but this happens in chrome / ie / ff / opera.