Hardware freezes (possibly memory?)

My computer has started ‘freezing’ and posting up BSOD, the BSOD always relates to 0x00000a caused by ntoskrnl.exe but freezes are more common than the BSOD.

A day ago the OS told me it had “critical error Windows will restart in one minute” Which it did, and it corrected a corruption of the O/S.

The freezes aren’t random, have to reset the computer by the power button, they can be triggered, processing a video in Corel VideoStudio for example freezes it within seconds, Prime95 causes an instant freeze, but playing Assassins Creed 3 doesn’t and it plays flawlessly.

I ran memtest86+ the first time I got a report of test six failing but then I ran it again for two hours and wasn’t able to get a fault (I plan on leaving it running over night)

Am I right in thinking this is a memory issue? I haven’t ruled out the power supply but my experiences with power supply failures are instant restarts (like a power cut) rather than a locked up freezes.

Being able to play Assassins creed 3 flawlessly has confused me.

Any help/info would be appreciated.

Intel Core i5-2400
Nvidia GTX 460 Super Hawk
OCZ Gold 4gb DDR3 1333Mhz
Samsung 1TB drive I think it's a Spin-point F3
PSU: BeQuiet Pure Power L7 530W

System was built by myself, it's about a year and a half old.