Could be overheating. Disconnect the PC from the mains and open the case. Use a new artist's brush (small) and gently sweep between the CPU's cooler fins while holding a vacuum cleaner nozzle nearby. Not too close mind and do the same with your GPU fan and any other dust settled on your circuit boards and case. Just take your time and try not to spin any fans with the vac's nozzle. See if it helps.
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