Hi Darrel,
I have put this code at the beginning of the program (really there is some register inizialization before):

debug "RCON ",#RCON," - STKPTR ",#STKPTR,13,10
RCON.0=1
RCON.1=1

When I power on the board I see "1 RCON 28 - STKPTR 0" so POR and BOR bit are cleared.

Then I leave the board running and when the problem occours I have a reset and I see "RCON 31 - STKPTR 0".
What does it means ? I have difficulties to guess.