All interesting points and all taken on board.
I did do two installs. Some video editing software and a USB 2 both with their respective cards. The former was a straightford install. The latter did need drivers, which I installed. But the shutdown, I should say re-starts, were very sudden and i played no part in them.
The curious thing is that each re-start took the start-up a stage further each time, until it then settled down and didn't re-start and it's been like since.
One further point. I did do a de-frag prior to the shutdowns.
It is absolutely fine now btw.