Imac will not update from OS 10.13.1

My hardware is a late 2013 Imac with 3TB fusion drive. There is a boot camp partition as well on this drive. All attempts to update from 10.13.1 fail - including: combo update from app store, combo update downloaded from support site - both 10.13.2 combo update and 10.13.3 combo update. In all cases failure includes a long (60+ minute ) hang during installation followed by a restart where the restart screen flashes on and the computer shuts down and restarts again in an infinite loop. Booting into safe mode does the same, as does single user mode. booting into recovery DOES work (thankfully) but reinstalling OS results in the same issue. The only thing that works is to reinstall back to the old OS using Time Machine recovery. I have now had to recover my machine 3 times with similar results - any suggestions above what I've already done would be helpful.