Computer freezes after random time of work

[PhunkyMunky]

Hello I have the following box:
CPU: INTEL Q9550
VGA: ASUS RADEON HD4890
MB: ASUS P5Q
RAM: 4GB 800MHZ DDR2 NON-ECC CL5 DIMM KIT
HD: WD CAVIAR GP 500GB SATA2 16MB INTELLIPOW
PS: CORSAIR PSU 650W 12CM FAN ATX PFC 80+
CD: SMG DVDRW 22X BLACK BARE SATA
After few months of perfect running my PC started to freeze at random times. It can be from few min to hours. After freeze I can see last screenshot in the monitor but nothing is responding and some of the PC fans are working faster than before freeze. Sometimes computer freezes during start up even before OS is started to load and even with cold box so I doubt it can be OS problem or heating...
I have checked all contacts inside box.
I have cleanly reinstalled OS to Windows 7.
I have tried launching Windows memory test. If I run standard tests it completes with no errors, but if I run extended tests after running random time computer freezes, everything stops responding. I have 2 memory sticks – have tried to remove one rerun tests – it still freezes, did same with another one still freezes.
I have tried running windows scan disk. During the scan computer freezes at random times too. Sometimes it finish without any errors on smaller disks.
I have checked temperatures. System 40-50C CPU 40-60 GPU 50-70
I have also noticed that after computer freeze and after I restart it – it hangs on initial start – the only thing I can see is dash on my screen. If I restart it again it boots up normally.
The only hardware I have changed in my box is CPU cooler. Change was from stock one to ZALMAN CNPS9500 AT. Everything looks ok with it – temperatures are about 10C lower then with stock cooler.
I would be very thanking if someone would list things that could help me to detect what is causing such freezes of my pc.

[Xylon]

Heating is not problem there may be problem due to to some incompatibility between motherboard bios and windows 7 so may be you need to update bios. but first check for all hardware connection like ram in slot also try with using your onboard graphics card

[PhunkyMunky]

As i said pc sometimes freezes ouside OS so windows does. Nothing here. I have also tried diff video card and still have same problem