Be very carefull here. Have a close look at the processor and make sure it's heatsink is correctly fitted. It must be firmly seated on the processor. It's easy if your not carefull to catch one of the corners of the socket so it's not making contact with the chip. This is exactly how the system would respond under these conditions. It will sence that the processor temperature is rising too fast and switch its self off after a few warning beeps.