UPDATE: no crashes for a month ... but then it returned. First on Skyrim and then it spread over a couple of days to other things. Lots of debug later, I established that positioning a portable fan blowing on the POWER SUPPLY INTAKE stopped the crashes. I've now replaced the power supply and all is good. Why Skyrim? Perhaps because it puts load on GPU and CPU and combined they push a marginal supply over the edge. There is no obvious clue in HWINFO except 12V droops to 11.75, not conclusive