Does your game use threads at all? I have come across problems before with multi threaded applications and hyperthreading cpu's, best case you get a slow down , worst case a full on crash (with no exception raised, the app just bombs out). I have found the best option is to disable it. There is a method through task manager to force an application to use only one cpu (cpu affinity I think).

In my case the problem was down to the communication between two threads.