PC problem: black screen, crash, fan speed up.

Hi, i've been having a problem lately with my pc, randomly (i cannot reproduce the problem, i tried stressing the cpu and the gpu with no result) the pc crashes. It happens in two forms:

1. the screen goes black and the gpu fans speed up, i have to manually restart the pc.
2. the screen takes random colors (usually a solid color or a color with some stripes) and the pc restarts by itself after a short time.

I thought that this could be a problem related to the gpu or the psu, so I tried putting my gpu in my brother's pc and using mine with the onboard graphics. Since then (about 1 day and a half) neither of the pcs crashed.

I'm a bit confused about the problem, I still think it has to do with the psu but i'm not sure and i don't wanna waste money buying the wrong part.

My pc has about 2 years and a half and it never had any problem since i built it.

Here are the specifications (in case they are needed):
CPU: Intel i5 3570k
PSU: Corsair tx 650
Graphic Card: Gigabyte Amd Radeon HD7950
Motherboard: Asrock Z77 EXTREME4
RAM: 2x4 GB Corsair Vengeance
CPU Cooler: Cooler Master Hyper 412S
Hard Disk: Western Digital Caviar Black 1TB

Do you have any idea on what could be the problem?

Thanks.

Ps.: while stressing the gpu i checked the teperatures and seemed normal.
Pps.: i don't think it's a software problem cause the problem happened either in windows and ubuntu.