Last Problem: Cannot reboot Aurora immediately, must wait

I had an Aurora that would not boot at all. I replaced the motherboard and the memory, and, after some effort, got it working OK. With one exception. When I turn the machine off or try to reboot it immediately, it will not restart right away. The fan will start at a high speed. with some chasis lights on, but the computer will never display video. Or sometimes, the computer will start with a normal fan speed and then shut off and restart at a higher speed.  The only way to get the computer to boot properly is to wait for 15-20 minutes before attempting to turn it  on again. Is this a faulty BIOS issue? I thought so, so I did a BIOS upgrade from A10 to A11, but the situation is the same. What else could it be?