hi,
since a couple of months my pc sometimes shuts down without a bluescreen, i installed the pc new and it works for 2 months now i want to play Test Drive Unlimited 2 and when i start i see a screen for 5 minutes than the screens say no signal, GPU´s fan slows down than up. Sometimes no more pic is shown and sometimes i can play for 10 minutes and than it says again no signal and after that it shows nothing, if i want to go on the desktop after the first no signal the second no signal screen comes instantly.
I have already test another ram but it wasn´t the ram
 
what could than be?
 
(single gpu is runing i testet both cards in tdu in singlemodus and there is no problem)

Did you install the card with the power off.  Also try different power connections off the psu, it could be faulty.

can you told me how you mean this with the power down each time i don´t understand it 100%.

Well I hope you did not install the card with the pc powered up. You just plug in, power up and then install drivers. Then you should be good to go.

jes´, i plugged in cards install driver but surely before i put on the powerplug ;)

Test the cards separately take 1 out test the other switch them then if they both work separately i would try a different SLI bridge.  it could be the SLI bridge is bad or 1 of the cards is failing.  From the sounds of it one of them is overheating but it could also be a bad bridge.  I had a bad flex connector that would cause the fans to ramp way up on POST.  Switched it out for the hard 3 way connector didnt have a problem after that.  Well until recently when the VRMs on one of my 570s burnt.

mhm but i think its a hardware problem because if i turn the pc on, sometimes it does´n start the mainboard don´t make a noise and the screen on the mainboard doesn´t show anything

So you press the power button and no POST?  I would look around in the case make sure nothing is shorting out.  Make sure the leads on the back of the power button are not touching any metal.  Make sure the wires are not damaged in anyway.   Do you have a spare PSU you can try?  it may be power related.  Or a DMM to measure the 12v rail with?  if you are dipping below 11.5v or so it might be causing the no signal problem also could be why it fails to post sometimes not enough power to start.  I would verify theres no shorts then look at the PSU might be a problem with it.