Hi, just to add my personal experience. I had the same symptom: FLBK LED flashing 5 or 6 times, then fast flash, then starts again this cycle, and nothing else working, cpu fan not starting, etc...
I tested the board outside the case as recommended above, with only the CPU+CPU-fan and nothing else (no RAM, no HD, nothing plugged except power for the board and CPU). It was ok, CPU fan started and board showed red LED because of no RAM module found (which is normal).
I put the board back in the case and I tested it plugging all components/wires one after the other (power off when plugging something, of course). Eventually it was the case LED to show Hard Drive activity that was creating the shortcut.