i'm almost positive it's not your isp. i have the same problem... it's with a time_wait in the networking layer causing it not to be able to open the right port... i have no idea ow to fix it but i'm sure of the cause... the game will work spurratically but you most certainly will haave "not responding errors" often...
if anyone reading this can think how to manually close a link in win2k ... i'm all ears... but after trying to join a game for a few mins i have bout 20 time_wait connections open and they clog up the avail ports.
email me if you know the answer
StormTide@home.com