I'm guessing most likely reason is thermal cut off, maybe due to dust or damage from storage. It doesn't feel hot and at least one fan seems to be going OK. So I thought I'd open the case and have a look and then maybe also install a temp/fan monitoring app like HWiNFO or HWMonitor.
Does that sound like the right approach? Anything in particular to look for inside the case? Is there a dedicate fan for the graphics that could be faulty? Anyone have experience with any of the monitoring apps? Anything else this could be?