iCloud Photos process restarting continuously
I think I've resolved this by installing iPhoto (fortunately I had paid for it before when I wanted to use it before changing my mind), enabling Photo Stream in the iCloud Systems section (without iPhoto I couldn't do this), leave it for a minute then disable Photo Stream.  The iCloud Photos process disappeared from Activity Monitor and has stayed away.

It's odd that the preferences could get messed up if you don't want iCloud, but sometimes writing the in-correct settings followed by the correct one clears the bad setting.