Windows 7: FPS and sound stuttering + nvlddmkm BSOD

Sorry for the weird title, i don`t really know what the problem is..

First, the stuff that`s happening:

- while playing any game, there`s a lot of stuttering in the fps. And every second or 2, the screen freezes for a like half a second, and the sound crackles and stutters.
So it`s like, fps stutter -> freeze for half a second with audio crackle -> fps stutter -> etc. ( i hope that makes sense! )

- i`m pretty sure that audio crackle happens when the cpu load is high, because it`s not limited to games. If i`m watching a video or listening to music, and do something like open a folder or another application, the audio also stutters for a second. It even happens in the windows welcome screen!

Also, i monitored the cpu usage while playing. it`s around 50% during the game, and and there are spikes to 100% at each of the audio stutters.

- sometimes, the games crashes to desktop with the "display driver has stopped responding and has recover" error.

- Also sometimes when i boot up the PC, the screen stays black before the welcome screen, and then i get the nvlddmkm BSOD.



This all started suddenly, and i have no idea what caused it.
The important thing is, that problem started 3 months ago, it lasted for around 4 days, and then went away, and i have no idea what fixed it. And it came back yesterday.


Here`s the stuff i tried so far:
-reinstall windows.
-different GPU, and sound drivers.
-disable Lan card.
-disable sound card. (of course when i do this i don`t hear the sound stuttering, but i still get the stutter with cpu spikes thing.)
-All the temps are perfectly fine.

I`ve been searching for 2 days and can`t find anything, i really hope you guys can help.

Oh, i forgot to add my specs.
Q6600@2.4, XFX 9800GTX+ 512MB, 2x2GB DDRII, Gigabyte EP35-DS3L