I had a similar problem with an A7V8X-X board. In my case though the system
would power up for a second or two and then it would shut down. I had to
either remove the power cord or to press and hold the power switch for at
least 4 seconds so that I could try powering up again. When it finally
powered up, it stopped at the CPU Speed Selection in BIOS saying that the
previous boot failed because of an improper CPU speed.
In the beginning this trick worked but then it just took longer and longer
to get the system running. Once the system started I had no problem using
it. I had it powered up for 10 days without any problem.
Restarting also worked. It was only the cold start that gave me problems.
Even though it looked like the power supply was good (just because there
were no problems for 10 days or at restart), it turned out that the culprit
WAS the power supply. I replaced it and everything started working again.
So if you can, try a different power supply and see if it makes a
difference.