Tomorrow will try PSU and clean memory slots, blowing out CPU.
The 16min 12sec failure during Memtest is now stuck (i.e. it never gets past the first pass, sometimes errors are found, sometimes not - it seems) This is repeatable from cold or on repeat re-boot so can't see it being a temperature thing. This seems to be the strangest part. Any more ideas or should I just give in and buy another?