system
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I uninstalled Malwarebytes Anti-Malware Free and Repaired Avast once again but the problem continued.
So I backed up my Avast settings and uninstalled then reinstalled Avast, and imported my settings back in.
For the last day or so, I have not noticed the Avast service shutting itself down.
If I do experience the problem again, I will uninstall and reinstall Avast again but leave the default settings initially to see if that resolves the problem.