If it's restarting, it could be due to a system fault and will require changing a setting to identify what is happening.

If it's just shutting down, it could be due to overheating. First thing to do is check that your fans are all running and that they aren't clogged up.

Disconnect from the mains, take the case off and inspect inside. Clean carefully, I use a brush attachment on a the vacuum cleaner, but some would say I was living dangerously.

Then plug it into the mains and start it up - watch to see the fans are spinning at a fast speed.

Disconnect from the mains again when you replace the case.