No specific program being used and it restarts.
But in this case you have to unplug the Power cable and plug it back in for the PC to be able to restart again.
Any restart intermittently problem is 98% of the time hardware. In 20 years of troubleshooting its been software only a handful of times.
1/For complete random restarts the highest possibility is that its a RAM block. If you have more than one RAM block in the PC you can just remove one and run until it crashes again to eliminate. (otherwise you have to get another block of RAM to troubleshoot and that can be frustrating if its not the problem.)
Other Reasons for restarting intermittently can be:
2/ The CPU and the Heat-sink have disconnected. Since about 2010, we have found this to be quite common. Either Thermal Paste has declined in quality or CPU’s are running a lot hotter (likely the latter)
You need to remove the Heat-sink, clean the thermal paste off it and reapply some more then reconnect carefully.
3/ Power problems can also cause this, it can be the power supply itself but it also can be the unused power cables from the power supply creating a short, usually with dust involved.
For this particular problem it was option 3. The detail that is different from the other options is that you have to remove the power cable before the PC will start again. We cleaned the dust from the Loose cables from the power supply and made sure none were touching the housing of the Case and it didn’t occur again.