Unstable pc working with 4gb RAM

Hi, one day my PC started to act very weird. What I mean is sometimes I can play half a day without anything wrong, max. settings, etc., but sometimes my PC is lagging very much, like 1-2 fps. And games do not slow down time by time. They start slow, and just continue to play slow. Most of the times restart helped, but as mentioned, not always. Installed newest drivers, reinstalled windows for several times, both 32 bit and 64 bit. No viruses found also. I have suspition that this could be the fault of the RAM, but not sure. Earlier I had problems with both mem sticks inserted (2 x 2 gb ). After I removed one stick, problem was solved. Then I was suggested windows 64 bit, but problem remained. I tried swapping slots but nothing helped. Also tried changing PSU, but problem remained.

Here more tech info:
Intel Core 2 Duo E6750 2.66GHz
Intel DP35DPM Socket 775
A- Data DDRll 2GB PC6400 800 MHz X2
GeForce 8800GT 512MB
PSU 450W

Windows Vista Ultimate 64bit SP 1

My pc voltage setings is :
11,9
5
3,3
1,2
Unfortunately i dont know what timings should i set.