I can join a server fine in MineCraft (be it browser or client), but when I'm in I will usually lose connection after 5 seconds.  I'm able to play for an extra minute before it tells me I've lost connection, but I can't type and nobody moves.  My connection is fine in any other game (except for Left 4 Dead 2), so I have no idea what the problem is and how to fix it.

Any help?