Answer was provided by phantomfoot on App Inventor forum - Error 1103 indicates internet is not available. And thus is easy to see why it is intermittent. His solution within App Inventor 2 works perfectly. From his post on App Inventor forum:
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Error 1103 is no internet connection.
https://groups.google.com/d/msg/programming-with-app-inventor/mc3YBXbpQ1s/SklzSh17Qp0J
That would explain why it is intermittent.
So one way to program for this is:
Catch the error in Screen1.ErrorOccurred.
If errorNumber = 1103,
notifier1.ShowAlert(“No internet connection detected. Retrying…”)
then call POST_Call_Function again. (It will then keep repeating calling this function until it connects).