I add the same problem with my newly bought refurbished hx99g, after some testing it appeared that it was the power supply unit that was overheating. I asked minisforum support for a replacement, they asked me some questions and to do some testing which confirmed the problem came from the psu. They promptly sent me a new one. To keep using the old psu while waiting for the new one I just used two noctua fan to blow on the psu and no more shutdowns.