When I start a game (I play Assassin's Creed: Brotherhood, Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops) on my Windows 7 64-bit computer the screen freezes after at least five minutes and the sound usually goes into a loop. The screen becomes black (out of signal), whereas the game itself seems to continue; after a few seconds the screen becomes normal again. This repeats itself on short intervals until the game finally freezes. When this happens I have to exit the game using the task manager. The game also freezes immediately when I try to change the resolution. The "spontaneous" freezing however happens more often (and it starts to appear faster) when I'm playing the game and am not only in the menu.
This is after I installed the latest graphic drivers. Prior to that, when I had the now second latest graphic drivers the screen went out of signal and the sound went into loop (just as it does now) but this didn't fix after a few seconds but restarted my computer instead; the first time it happened, it didn't start happening in intervals.
When playing Assassin's Creed though, I do not experience any problems. Prior to intalling the latest graphic drivers the screen (when playing Assassin's Creed) from time to time blacked out, but got normal again and didn't cause any major trouble.
What do you suggest me to do? I have checked some forums and found a rather old post where someone had similar problems. He sais that the motherboard's sound drivers were to blaim: Post: Problems with Games with 64 bit (he didn't have an ASRock motherboard though). I would appreciate some help, as I do not know what to do.
Thank you!

Motherboard: H61M/U3S3
BIOS: P1.10
CPU: Intel i5-2500K
VGA: Nvidia GeForce GT 430
OS: Windows 7 64-bit