Unknown problem, Possibly PSU

[Vertiginous]

One of the computers in my house has recently begun playing up and it will sometimes lock up and become unresponsive and the only solution is to reboot, We tried to determine the cause of the lockups and so far have had no success, We ran Prime95 and Memtest and ran a gpu bench mark and so far all of the components have held up, The computer has been formatted and the problem is still there, We suspect the PSU is at fault but we cant tell for sure, The psu is a OCZ ZS Series 750W '80 Plus Bronze' Power Supply

CPU: FX6100
GPU: 560 Ti

any ideas on what it could be?