I own a P7P55D-E Pro board and the board does take some time to start the POST.
What I did come across is that after adding all my drives and GPU's etc... I blew the power supply 500W this might be you case to as i don't know how many devices you plugged in. Another test you can do is reset the BIOS. Also exchange memory if you are running more than 1 module. Also make sure the memory is seated properly. In most cases it is the PSU that goes 1st. Lastly try replacing the GPU.