CPU temp.
CPU temp.
I began by testing AC Valhalla. During the first session it ran smoothly for nearly two hours, keeping the temperature around 40°C. After another two hours I restarted and within a few minutes the temperature jumped to 70°C, prompting me to stop. About twenty minutes later I tried again and it stayed in the 50s°C for roughly an hour. It seems the issue is consistent, similar to what SamClan and MikeCoon reported. I expect it will require replacing the unit, which will take a few days, so I’m marking SamClan’s answer as the solution. Thanks to everyone who offered help and useful details.