Time for some MemTest burn-in.
mnem
not mnemTest... that's something else altogether.
Actually it seems it wasn't what I expected although the hint you gave about rolling back W10 updates got me tinking............
After much shagging around in which it got close to Bob Pease flying lessons a couple of times I stopped the boot in Bios and let everything cook for a while yet after some time it went into reboots again
so ripped the Data drive out and took it to our family PC to ensure the last days work was saved and safe
and then threw it all back together after eraser and IPA work on the RAM pads and let it cook while stopped in Bios where it now ran for an hour without issue.
Then let it full boot into W10 and another hours cooking where it seem just fine.
So now it's back in its normal spot allowing me to type this reply.
The only thing I can put this down to is last evening when putting it into Sleep mode noticed the HD light was showing some activity which has never worried me or it for that matter before but it may have been working on one of them silent W10 updates and sending it to sleep in the middle of it may have forked something.
Anyways my head hurts now from all these carry on's not to mention all the oogling at new $ PC's and $ PC bits and $ bobs while waiting for GB backups to finish.