Thought I would give you guys an update. The search for ISRs that were of too high a priority has not been fruitful but I have narrowed the problem down to a fairly innocuous looking line of code. Line 97 below is the course of the problem. Line 101 works fine. uChargePercent is of type uint8_t.
In the end, this seems to have been a stack overflow problem, although I do not appear to be receiving an indicator from FreeRTOS. I have not looked at how exactly FreeRTOS is monitoring stack usage but, in other RTOSs, I have observed a similar issue:
If the OS is monitoring watermarks, a string utility such as snprintf may actually modify values in an adjacent stack without changing the watermarks in its own stack. This may or may not be the case here. Anyway, an increase in stack allocation for the task corrected this issue.
Thanks for the help.