random BSoD

Getting IRQL not less or equal BSoD after running my computer for a day or 2 the problem is it doesn't have a problem if i leave it running memtest.

also if restarted straight after the BSoD it will BSoD before windows even finishes booting.

tried test with UBCD but not finding any problems.

anyone had a similar problem? or any other ideas.

Operating System
Used XP/Vista and even 7
Drivers
drivers are all upto date after a fresh install
CPU
Intel Core 2 Duo E6750 not over clocked at all
RAM
Corsair 2048MB (2 x 1024MB) PC2-8500 1066Mhz DDR2 RAM - 5-5-5-15 - Dominator Series (was replaced recently as old MB and RAM had same problem)
Motherboard
Gigabyte GA-EP45-DS3L (was replaced recently as old MB and RAM had same problem)
Video Card
Geforce 8800 GTS
PSU
Antec EarthWatts 500W ATX Power

If you had the same issues with your old motherboard and RAM, and it only happens after a day or so, I would be suggesting a graphic card issue, possibly overheating or draining the life out of your PSU, I would try a different card for a few days and see how you go.

So you've tested RAM? Also does it give a particular file name when it crashes at all?