I believe I found the problem.
It has to do with the deletion of a file previously put into Avast's exclusion list. When I installed Avast and did its first scan, it signaled that I had a trojan in three HOTKEY script files I created using AUTOHOTKEY's package to create hotkeys. To verify the findings I wanted to send the files to
www.virustotal.com which required me to put the files in a separate file other than the Avast quarantine file so I could upload them. Thus I created a file SUSPECT and put it in Avast's Exclusion list so it wouldn't scan it again. Later I deleted the file completely, but did not remove the file from Avast's exclusion list and thus in its AVAST.INI file. After removing the file name from the Exclusion list (I did this by editing the AVAST.INI file and deleting it there), I no longer have seen the shutdown problem to-date. Have restarted the PC over 10 times to test it. Hopefully this resolves my problem forever and can help others.