So for the past year or so I have been having problems with my computer crashing..
I was certain that it was an overheating problem ( and it was) so I got a very good heatsink and put everything in a new case.

For a while things were fine, but then it kept crashing. I had a fairly old psu and thought it could have been going bad so I bought a new one and it ended up not doing much(at first i thought it did)
I would like to note that my computer doesn't crash often..it's pretty rare(for a crashing problem). Happens maybe once or twice a week. Usually when playing a game. Recently it has just been turning off, not freezing, but i've had freezes occasionally.

specs:
case: Corsair Carbide Series 200R
psu: EVGA 750 GQ, 80+ GOLD 750W
os: Windows 7 Home
mobo: ASUSTeK Computer INC. P8Z68-V PR
cpu: Intel Core i5 2500K @ 3.30GHz
ram: 8.00 GB Dual-Channel DDR3 @ 668M
ssd: 117GB INTEL SSDSC2CW120A ATA Dev
card: asus AMD Radeon HD 7870
also have a 1tb hitachi hdd

The oldest things are my cpu/mobo and graphics..i can't figure out what is causing this

No bluescreens?

Reseat rams
Reseat gpu
Reseat all drives from sata ports
Update mobo bios



Did you run any heavy applications when freezes / shutdosns occurs?

Is anything overclocked?

nothing is overclocked. i actually tried it for a bit and had more problems.

no bluescreens at all. may have had ONE bluescreen in the past year.

i recently upgraded my case so nothing needs to be reseated.

i'm usually gaming when it occurs, but it has happened when i'm just lightly using my computer too.
Can't be overheating. I monitor temps and they have been much cooler since I installed huge heatsink

Run prime95 or other stress tests. If you can get it to shut down remove parts one by one such as you, ram (switch each stick and slots), any extra hardware besides mouse/keyboard.

This will start to track down potential hardware that might be causing the problem.

If prime95 or h264 benchmarks don't trigger it then try your games.

From the sounds of it since it happens so rarely this will be hard to find.

You can also check the windows event manager to see if there were any problems.