I'm betting your CPU fan has most likely seized up. This is very normal to happen if you tend to leave your computer on overnight often or your computer is pretty old or both.
The oil on the spindle of your CPU fan has probably run dry from excessive use. (This happened on my old desktop computers power supply fan eventually it seized up and sometimes when I booted the computer the fan would sometimes start spinning and sometimes it didnt it often made a low grinding noise which is an obvious sign the fan has reached its end) When this happens its better and easier to just replace the fan. They arent really that expensive to replace.
EDIT: If you can monitor the fan until it cuts off if the fan is still spinning quickly when it cuts off its probably a power supply issue.
If it was hidden processes causing it which is highly unlikely you would easily notice a massive performance drop in your computer. 81c celsius even on a 100% load cpu is really quite rare even if the ambient temperature is over 40c
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