I'd appreciate your support to this minor problem.

As I play Sins, on and/or offline, the graphics may slow down for a few minutes (frame-freezing, jittery) and then it gets back to normal again. But about ten minutes later, it happens again, and goes like that over and over.

My PC desktop is just one year old. Gateway GM5478, Window Vista 32, 2.4 Core Quad (4096 cache), and GeForce 8500 GT.  (Dxdiag available upon request.)

(I must be honest, though, that this also happens on Medieval II Total War. I've already taken my PC to repair, but they say hardware works fine; no virus, bots etc.)

Any suggestions on how to solve this issue are very appreciated. Thanks.