I think ArmA isn't handling the Dual Core (Core 2 Duo or AMD FX & X2) platform correctly, it may be simple as that.
Quote from Wikipedia
Almost everyone who's having issues with ArmA in this topic are running the game on a dual core platform. And most of those writing they have no problem are using single core systems.
Quote from Suma:
And Chris Death:
Is there a way ArmA developpers could build a patch to support these processors correclty?