Have you changed any components recently? Either the power switch lead is on the wrong pins, or the power supply may be too weak. Try removing and reconnecting the 20 or 24 pin atx lead and 12v lead off the ps to the motherboard. I also had a similar problem ocassionally on a new board. It disappeared when I changed the bios back to the MSI post screen, which is the default setting. Same thing for a similar biostar board.