Hi, joined after googling the OP's issue. I have a portable LAN PC built off an ASUS micro ATX board, 4GB DDR2, and an Athlon X2 5400+ (non-OCable and running at stock IIRC.) I have had a Radeon 5750 to which I attributed this exact issue, until I replaced it with a 6770 and the issue returned after a few minutes of play. With the old card I tested this on League of Legends, BF 3, FF14, SW:TOR, and now Guild Wars 2.
Both cards are DirectX 11, but this clearly doesn't look like a hardware defect. Old card has been tested in other machines with no issue. New card only tested with GW2, and the slowdown seems a little less pronounced, but it's the same cycle. A minute or so of good gameplay, 5 seconds of garbage.
Sorry if bad manners raising this old thread. Anyone have any further advice? I'm going to try replacing the processor.