I ended up spending yesterday totally reformatted the iMAC, restored data and reinstalled applications(after arlier attempts to reinstall only Big Sur... did not solve the problem). I did find at least two problem applications but these are working fine on a MacBook Pro I also upgraded Big Sur from same release of Catalina. The MacBook Pro still is running with no issues after its upgrade, although to be safe I have also removed the problem applications from this system as well.
So far so good. I assuming problem solved.