Game Freezes and sound glitching.

HI all.
lately i have been having a problem with high end games crashing after a certain amount of time.

what happens is the game usually freezes and the sound on my pc will start glitching, the game will usually crash to desktop, and sometimes but not always there will be some graphical artifacting. like icons having colours in the wrong place or being muddled.
also its all sound that glitched including music from things like media player.

this only seems to happen on games with high graphics.

it started happening when i updated a few components of my PC(Cpu,Ram Mobo) so i thought one of these may be faulty, tested Cpu ran prime 95 for 6 hours and no errors.
ran memtest overnight and no errors. don't know how to test a motherboard though.
Gpu was running a bit hot so cleaned it and added new thermal paste and it now runs alot cooler but the crashes still happen.

with the graphical artifacting that sometimes happend and this only being on highend games ai naturally assumed it was gpu overheating. but my previous experiences of an overheating gpu coupled with the fact its not running hot make me think otherwise.

does anyone know what may be causing this?
its starting to annoy me.


Specs
Amd phenom II x4 970 (3.5ghz) not overclocked.
4gb corsair xms 3(2x2gb sticks)
asus m4a78lt-m
geforce 9800gt xfx black edition(comes pre overclocked and never had any trouble with it before the new components, but this may be due ot my old cpu throttling it and not letting it reach max capacity)
thermaltake 550w Psu(not amazingly powerful but should be enough for the coponents.)
Windows 7 64bit(also got the crashes on 32 bit)

games that crash.
Battlefield bad company 2
Gta 4
darksiders( not a particularly graphic intensive game and it takes an hour or two for this one to crash.)
most crashes happen after about 10-30 mins of gameplay.

i think it has something to do with games with high end graphics, since i can play WoW for hours without it crashing.
with the sound glitching is it possible its a sound driver issiue ?