Here is what just happened today:
I got a desktop dropped off at the shop which had a problem of shutting itself down after about 5 to 10 minutes.
I checked the power supply, the fans, the RAM, the hard drive, disconnected everything except the basics and still it would keep shutting down. Finally, I was going to change the CPU to see if that would help, and I found out that one of the clips connecting the heat sink to the CPU was broken. I found another heat sink/fan just like the one that was defective, changed it, and restarted the computer. The computer has been running fine ever since. Obviously, without a tight connection between the CPU and the heat sink the CPU was overheating and shutting down to protect itself.
This was a new one for me! Some one had broken the tab off the connector. The power supply had been changed previously, and that might have been when it happened.
You just never know until you do a thorough troubleshoot!
Professor Randy says: The joy of the troubleshoot! Go through every step until you “solve the puzzle”, then you will learn your craft like no other!