Just built a machine based on a Gigabyte P55A-UD5P board with an Intel i5-750 cpu.
On powering up the Phase LEDs all light up for less than 1 second then the power goes off. This cycle repeats every 5 seconds. It doesn't get as far as a POST beep.
It behaves the same way with or without RAM or graphics card installed.
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Do you have a power supply to swap and test? I have seen a similar issue and it just turned out to be that not all the power cables to the mobo/gpu were seated correctly, so that's one thing to check real quick.