I have been having random restart and shutdown problems with the Arcade PC for a while now.
Diagnostic software indicates that there is nothing wrong with the RAM, and considering that the power LED to the X Arcade controller often craps-out, I figure its an issue with the power supply.
The current Power supply unit in the Arcade PC was 300w, so I went out to the Computer Fair and picked up a 550w unit for under £20. I put it in, connected it to motherboard and other hardware and have not had a shutdown/restart since!