Just as a matter of interest, I had a similar reboot loop, turning on for 0.5 second turning off for 0.5 second.
I had a friend with same motherboard and one unusual difference was the CPU Clock DRive and PCI Express
clock drive voltage,
The CPU clock drive was accessible from BIOS under MIT , then advanced frequency settings,
On my board both values were set to 700mV.
On my friends machine CPU clock drive was 800mV and PCIExpress Clock Drive 900mV.
In fact on page 37 of my GA-P55A-UD3 motherboard manual, the printed values were
the same 800 and 900mV respectively.
I changed these values and so far system has been stable.
It may well be worth checking these values against what is printed in the motherboard manual.
Again I have different hardware an Intel i5 750 so values not forced to be same, hope that helps.