I've had this happen more frequently before I switched my router.  But basically, I can play a game fine, and suddenly my connection goes terrible, and I start getting some extreme flakiness.  It drives the game into an unplayable stutter, so I usually end up quitting.

I had BitMeter 2.0 running in the background this time, and I captured a picture of my network traffic.

The left side is the game running normal.  Green is Upload, Red is Download.  Yellow is where they overlap and are equal. Hence the left side is mostly a fixed amount of yellow - which would rougly expect in a P2P game.  Then about a 1/4 of the way to the right, I hit 'crap' mode and the game went all spotty.

Anyone else have this sort of thing happen to them?  I'm trying to figure out what causes it.  It's possible this was a result of the roommate downloading this time through, but it used to happen without that.