GA-P55M-UD4 and Xeon X3440 - no post

Not a supported processor:
http://www.gigabyte.us/Support/Motherboard/CPUSupport_M...

I was reading elsewhere, that since it's 1156, i7 core, it should be recognized... No BIOS update for this one?

OK, have some updates, Xeon works just fine in these motherboards, highly recommended. My problem was (after a few hours of try an error) - stupid reset switch in the case, while connected to the MB, was not letting it boot! Found it by disconnecting everything by coincidence!

I'm having a similiar problem as you. I've build a pc with a ga-p55m-ud4 motherboard and an iCore i5 750. When I press the power button of the case, the board starts for 0.5 - 1 seconds and shuts down. After 2 seconds the process begins again. No post no bios... I've tried to disconnect the 'reset SW' cable on the board but it won't help. Do you have any advice? Thanks.

Re-seat your processor and check if any damaged contact pins, make sure the motherboard mounting holes did not apply any washers came with the chassis. Do not twist the power cable from PSU too much.