Your CPU is overheating. Likely because of accumulated particulates growing on the cooling fan over time and reducing its effectiveness. You will need a hobbyist’s utility brush and some Q-Tips to clean it out, or the bearings might be too far gone and you need to replace the fan itself.
Also look at the air intakes and fan exhaust ports and clean them out as much as possible.
Look for pet fur and human hair occluding the ports or fan as well.