Occasional Memory-related BSoD

Hi all,

I've just completed a new PC, and I'm getting some strange crashes on boot - but only every so often.

Usually when I turn the PC on for the first time each day I get a BSoD. After a couple of restarts it boots Windows 7 Ultimate just fine, and after that it can usually be shut down and rebooted with no problems. The PC appears to be functioning as normal once booted. Once it's been shut down for a little while and then switched back on I get some more blue screens.

I've tried moving the memory around, but it all seems to be properly installed, and when the machine does boot the memory is registered correctly by Windows. I've updated the BIOS to the latest version (F8) and tried both fail safe and optimized defaults. I was planning on reinstalling the OS, but it seems like a hardware issue?

Any help hugely appreciated. Thanks everyone.

Tom

Spec:
Gigabyte GA-P55M-UD2 iP55 Mobo
Intel Core i5 760
Geforce GTX275
Corsair 4GB DDR3 1333MHz/PC3-10666 (4x1GB, bought in two separate pairs)
WD7501AALS 750GB Hard Drive SATAII 7200rpm 32MB Cache (running in IDE mode)
OCZ Stealth Xtream 500W PSU

Blue screens: