PC temporarily freezes HELP

Not sure if this is the correct category to post in but anyways.

My system specs:
i3 550 (stock)
MSI H55M-E23
4gb 1333 Kingston Ram (2x2)
1.5tb Seagate HDD
Zotac GTX460 1gb (stock)
550w power supply
WIN 7 Ultimate (64-bit)

For the past few days my pc has been hanging especially Firefox. PC hangs (including mouse) for a bout 5 seconds then returns back to normal. Firefox hangs every minute. I don't overclock so I hope hardware is not damaged. I'm using AVAST and Spybot which detects no viruses or trojans.

This morning the PC froze and I had to hard reset. PC switched on but monitor displayed nothing. I reseated the CPU (put on new thermal paste) and the problem persisted. I dissembled the GPU, cleaned it out, applied new thermal paste and re-inserted. Same problem. After I reseated the RAM the PC started up again but still with the sporadic freezing.

Unfortunately I don't have extra parts to determine which part exactly is causing the problem and I don't want to buy anything before establishing the problem. My Idle GPU temp is 44 degrees celcius and CPU 30 degrees.

Formatting and re-installing windows is my last resort then I'm totally flabbergasted.

Thanks.