As there are no problems found in the diagnostics, this eliminates the possibility of any hardware failure. One thing that I am concerned about is overheating. In your previous replies you have not mentioned if you have observed any signs of overheating. If the computer doesn't overheat, this means the problem started after you did Operating System installation. Kindly, do the Operating System Reinstallation and check if the problem fixes.
In addition to doing the Operating System Reinstallation, please check the user's manual/manufacturer's website for the order of drivers installation and follow the same as sometimes incorrect order of drivers installation can cause some problems.
Do reply with results.