System won't boot with 4 pin +12V connected

[WiSo]

Build up a new system with the A8N premium, sharkoon silentstorm, leadtek 7800GT, Benq DVD and a samsun spinpoint.
System boots fine with the first boot. After a while it shuts itself down. On next boot I started to install windows. After a longer while it shutdown again. Afterwards I couldn't get it started anymore. The green LED is glowing but pressing the power button does nothing (no fans etc).
Even when disconnecting all drives (the 6 pin plug is still connected to the graka) nothing happens.
When I remove the 4 pin 12V plug the system starts the fans and the drives but the monitor is still black. Connecting the 4pin plug again still leads to no reaction.
Any hints? could it be the PSU or is it the motherboard?
I tried connecting my old ATX PSU with 20pin instead of 24 but this didn't work either.

 

[WiSo]

Forgot to mentioned that the 4pin plug delivers the +12V on both cables.

 

[mechBgon]

Welcome to the Forums. Please do us a favor and never refer to your video card as a "graka" again Owww, my brain!

Check that your heatsink is making good contact with your CPU and that its fan is running (duh). If you have any other 24-pin ATX2.0 power supplies with enough wattage, try them instead. Never heard of "Sharkoon," so that makes me suspicious that it might be more overrated poser junk, there's plenty of it out there.

If you don't have any alternate PSUs, try starting the system with a PCI video card if you have one, the goal being to try a card with low power consumption as a fact-finding step.

 

[WiSo]

Thanks for the reply. I'll try the PCI card when I'm at home. When I first start it I watched the hardware monitor in the bios for 5 minutes and the temperature was nominal (CPU ~32C).
The PSU was mentioned in a german technical magazin as quite with a good PFC.

 

[WiSo]

Update:
Just use the mainboard+PSU+CPU with no RAM nor video card connected:
With the 12V plug connected -> no reaction
Without the pin -> the voice says "CPU failure detected" which may be ok since the 12V are for the CPU?