I have seen PC's do that when they are overheating. When was the last time the dust bunnies were removed.
Use a can of compressed air to clean the board components and CPU heat sink cooling fins. A toothbrush works wonders on the cooling fan blades.
IF you're monitor is turning off but your computer is not shutting down and you're force to hard reboot it to get it to work again.....
I would suggest that you inspect your advanced power saving options paying particular attention to anything having to do with Sleep, Hybrid Sleep and or Hibernation.
The symptoms you are describing suggests that it is entering into a power saving state and it is not able to recover normally.
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