|So have you checked the system temperatures? Cleaning the dust out of the fan might not be enough. The heatsink might be clogged up and/or the thermal compound between heatsink & CPU might have degraded.|
Do you have to wait a period of time after the sudden shutdown before it will restart? A sure sign of overheating.
Boot up into bios and look at system health or similar section. Maybe under Hardware Monitoring. Usually there will be somewhere where CPU temperature is shown together with warning & shutdown thresholds.
You might also consider installing monitoring software such as speedfan
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