Compatibility Problems Causing Random Lockups?

This is my 2nd homebuilt PC and the 1st one that isn't working well. Here are the specs:

Antec 900 case
GIGABYTE GA-P55-UD3R LGA 1156 Intel P55 ATX
MSI R5850-PM2D1G Radeon HD 5850
OCZ Gold 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 Low Voltage Desktop Memory Model OCZ3G1600LV6GK
ZALMAN ZM750-HP 750W
Intel Core i5-750 Lynnfield 2.66GHz
Windows 7 x64


I am getting random lockups and reboots. I never get a BSOD it just either freezes permanently (I've waited up to 30 minutes) or reboots. I turned off the auto-restart but still no BSOD. I get the display driver stopped working message every once in a while.

Here's what I've tried:

1. Tried several ATI catalyst drivers but it doesn't make a difference
2. Replaced ATI 5850 with an old Geforce 7900 GS but it crashes too
3. RMA'd the motherboard and CPU to NewEgg - new board/processor has the same problem
4. Ran most of the tests on Ultimate Boot CD such as CPU, memory and hard drive tests - all passed
5. Disabled all of the Aero special effects - no improvement
6. Disabled IPv6 as I heard that can cause problems - no improvement
7. Reinstalled Windows more times than I can count - also tried Vista 32 but it crashed as well


My time frame to get a refund from NewEgg on most of this is in a few days. I really don't know what else to do. I'm tempted to send all of it back for a refund and just buy a Dell or something but I hate to waste my money on that.

Anybody have any suggestions or see any obvious incompatibilities that I am missing?

I'm at my wits end honestly. I've tried everything I can think of but I can't just swap out all the parts as I don't have extra power supplies or hard drives and so forth lying around. I guess I'll just have to return everything to NewEgg.