Random PC Crashes

So, my crashes are back, worse than before.

First off my situation: I have built my own Computer about a month ago, im not a complete PC layman, I do have some knowledge, but not that sick peripherals etc knowledge.

The freezes started instantly, there were 2 kinds of freezes, either a freeze, everything stopped, you couldn't move your mouse, all voices etc stopped so the pc really froze, or it was like the power was gone for a second and after a few secs the pc rebootetd itself.

Now the freezes are gon, but the crashes are back, usually about 1-5 times per day.

They occur completely random, while gaming, surfing, while I'm afk and literally not doing anything etc.

I tested the RAM with the built in windows ram tester and memtest86 + I repositioned the Ram slides, no change.

I took the gpu out and put it in again and made sure the contacts are clean and sitting right and that the gpu is steady.

I kept an eye on the PC Temperatures and I can completely rule overatheating out, not even the gpu gets over 45 degrees, rest of the system doesn't get hotter than 35°.

The PSU is not new, I have got it for around 1 1/2 years, didn't make any problems with my old mainboard, why should it now. I further did not have to take it out of the case when i put the new parts in so I really don't think its the PSU.

I have changed the old power cable because it was crappy.

Drivers etc should all be installed properly and performancewise my system seems to give me what it should, so no problems there, its not running perfectly, but its enough.

Ok so here are my specs:

Intel i5 2500k (not oc'ed yet)
ASRock z68 Extrem3 Gen3
ASUS EAH5870
8gb DDR3 Ram from Corsair
LG SATA BD-Rom
Windows 7 Professional 64 Bit
750 GB Western Digital HD
900W Corsair PSU (Got it from a special offer thats why the 900w )

I'm pretty sure that I connected everything properly and that all drivers are correctly installed, I have absolutely no clue why I got those crashes, allthough i would really appreciate your help guys.

If neccessary and one of you guys maybe know what the problem is, you can connect to my PC using teamviewer, if you can make my pc work fine again, it's worth it.

Also if you want you can add my ICQ and help me there (I could also post skype xfire etc but i guess ICQ is the most appropriate thing here and in all forums)

ICQ = 554458306

Would really appreciate any help.

I'm gonna to a 3Dmark 11 benchmark and post the results, maybe this helps you guys a little further.

Sry for any language mistakes, not my main language.

3rd one today >.<

I've had the GPU for 1 and a half year, why would it start making problems now?

The crashes just keep comming, 1 time its a freeze, one time a power blackout, and those crashes are just f****** my pc.

Is there noone in here who can help me? I haven't got a spare GPU nor PSU further I don't think they are the problem

Ehm is the outlet the cable that goes from the Power to the PSU or the cables that go from the psu to the hardware?

If its the first one, i changed that cable already.

PSU testers in my location cost all around 40€, is it actually worth it? Costs nearly as much as a new psu, and if the psu is not the problem the money is wasted either way.

Pc on, 2 minutes later --> power blackout