A system is going to shutdown for one of three reasons
- Power loss (obviously)
- Over heating
- Blue Screen of Death (BSOD)
Power Loss
This one is pretty obvious and other electronics would be off
Over heating
If they happen more frequently after the system has been on awhile this certainly an issue.
Grab a tool that shows temp sensors like speed fan and monitor the temps. CPUs can get up to 75-80C under load if it's getting in the 90C+ then you have an overheating CPU and the system probably needs to be cleaned and new thermal paste applied under the heat sink.
BSOD
These are mostly due to buggy drivers, but can also occur due to operating system corruption, or hardware problems.
If the system is...