cattledog:
My guess would be that in the rare, failing case, there is no null terminator.If there is no null terminator on receivedChars[] or tempChars[] I believe that the behavior of strtok will be undefined when parsing the last piece. The esp8266 also has a pretty short watchdog timer which causes resets if things get blocked
Why there would be a missing '\0' is a mystery.
The strtokIndx = strtok(NULL, NULL); resulted in continual crashes, no parsing possible. Next will be the A. test as well as a bare-bones program version (Robin2).