Computer randomly resetting

[abctmtom]

Ok so the computer I am trying to fix keeps randomly restarting. The power doesn't cycle like the fans and led and CPU fan are all still running and on. But the monitor goes black the then the mother board reboots. And I hear the mother board beeping and turning on. The computer runs 100% perfectly when it is working but sometime it just reboots randomly almost as if someone had just pressed the reset button on the case. I even unplug the reset button but the problem percistst. Anyone know what this could be.

 

[abctmtom]

Anyone know what this could be