This has suddenly been happening out of nowhere lately, but basically:
When I turn it on from a cold boot, the machine seems to power up normally, but then it shuts back down before the POST screen comes up. In between, I notice this red light flashing on the board itself near the RAM. But after it does that for a few seconds, the computer reboots itself and starts up normally.
The board is a P8B75-V; recent changes have included the addition of a rear fan and new graphics card. Strangely, Windows 8's hybrid boot also counts this abnormality as an "unexpected shutdown" in the Event Log (it also doesn't do the "quicker" boot after this happens), making me think that something else is up suddenly.
Anyone got ideas?