PC Freezing Up

Since I built my current PC back in January I've had problems with it randomly freezing up. The system goes completely dead, lights on the keyboard and mouse got out as well, and I need to hit the reset button to reboot it.

I can't see any relationship between what I'm doing when it does it. It's done it under pretty much all circumstances, from playing games to just being left idle on the desktop. It's also very, very randoms as it when it does it. Occassionally they'll be a load of random, static noise from the speakers. Just trash noise really.

Recently it's been doing it one per day perhaps every two or three days. Prior to this recent spate of freezes, it hadn't done it for two months! I can't think of anything I've changed or done differently that would start this new cycle off. But that's how it seems to occur. I can have a few days of freezes and then it won't do it for another few weeks. Obviously this highly random nature makes troubleshooting it very difficult. I can't tell if it's a hardware or software problem either.

So far I've checked the temperatures and they're fine. I've also tested the memory and replaced the PSU. Again all fine there. Event Viewer lists nothing that would suggest a cause for the freezes.

I'm really at a loss. I'm a PC technician myself and I havn't a clue what would cause it!

If anyone has any suggestion then I'd appreciate any help! Thanks!

Here's the PC's spec.

Intel I7-2600k
Asus P8P67 Pro Motherboard
8GB Corsair 1333mhz Memory
Gigabyte 580GTX GPU
Creative X-Fi Gamer Sound
Western Digital 640GB Sata-II HDD
Antect 750 Watt PSU
Windows 7 Home Premium