I'd say probably overheating. Check if all fans are clean and defenitely dont forget the cooling element on your cpu. Dust can sometimes really pack up in there.
If neccesary, use some toothpick or such to scrape densely clogged dust out between the ribs of your cpu cooling element.
Always be nicely carefull though because there's fragile hardware in your computer.
Watercooling is nice, but if it leaks ur screwed and unless you plan on overclocking your computer its kind of overkill.

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