PC freezes regularly (mostly during gaming)

Hi, I recently had a problem with my PC failing to boot on most occasions. I took it into a repair shop, where after a lot of trial and error, it proved to be a PSU problem. Whilst they were fitting the new PSU, I also asked them to put in a new hard drive and completely reformat everything and reinstall Win7 etc, because over the last year it had gotten a little buggy.

The original problem was solved by the new PSU, but the PC kept freezing regularly whilst I was installing my software or copying over my files from my external. The only thing I could do was a hard reset. I took it back to the repair shop who believed it was the new hard drive. They removed it, fitted another and again reformatted.

After that I only experienced 1 instance of the PC freezing (when I was installing my Adobe products).

However, whenever I'm gaming (BF3, Bad Company 2 or Left 4 Dead 2), the PC is again freezing forcing me to hard reset.

I'm at a loss really. I've got the latest nVidia drivers, new RAM, new Hard Drive, new PSU and have had my PC wiped clean (and phyiscally cleaned). I've never had this problem before. Anyone have any ideas? Could it be the latest nVidia drivers?

My spec:

OS: Win7 64-bit
CPU: Intel Quad Core Q9650 @3.00GHz
GPU: 2x Nvidia 280GTX SLI
GPU Driver version: 285.62
Ram: 8GB 16000
Sound card: Onboard HD
 
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