Pc freezing randomly rarely blue screens

pls help me diagnose fault my girlfriends pc is freezing randomly usually without a blue screen just totally freezes and has to be forced to power off then does dram cycle 3x then starts fine again pc is

ga-ex58-ud5p m/b
16gb g.skill ripjaw 1600mhz ram ( new )
gtx960 gpu evga ( new )
sata 2 500gc hdd i think wd its old but has passed 12 hr+ hdd scan checks
i7 920 processor not overclocked at all
500w psu cant think of brand but a good one not cheap chinise thing
running win 7 home premium geniune copy

any help would be appreciated

firstly i assumed dying ram Failed memtest86 so replaced the 6gb of xms3 but still freezes psu seems fine when testing with a psu tester and load should be around 400 so ok for the 500w psu.

Freeze is always a pain to diagnose is there any help in the event viewer logs that might help you? Think your post in the other thread of dying motherboard is the most likely problem but not guaranteed