Possible cause, now my system is running normally.
I tried changing a number of changes to Avast AV, reset defaults, reinstalled.
The problem is gone, I suspect I had a corrupt or too huge lscache.dat. I tried disabling temp ad permanent caching, rebooted, then enabled them again and rebooted. Writes to lscache.dat activity started at a much lower level and eventually fell to near zero. I note no unusual disk activity now. Hope it stays that way
I believe over 2+ years of system changes/updates and AVast updates, possibly lscache.dat became corrupt or overloaded with old garbage. lscache.dat WAS 3.6MB! It is 1KB now!!! It likely will grow with as I use the computer or do a full scan.
Perhaps resetting lscache.dat when Avast is updated or an option to reset it directly would be handy. This, of course, assumes I found the problem - a corrupt lscache.dat.