I meant flash memory where the program is stored in a pic.
The 16f84 I have had running a speedometer in a car for a few years always goes batty if the cab gets hot.
The chip is running, but the value displayed is all over the place.
This isn't any sort of proof that the chip is failing to read it's program memory though.
It could be any number of things like the program counter jumping all overt the place, but flash is still my guess.