hi,
if a program uses the internal flash memory only (here: both fetches code and read constants from there) then PSEN signal must not be at low level. Any time when PSEN goes low, it indicates about a program malfunction - either PC is overflowed or MOVC tries read from address over upper memory limit. So connect a flip-flop on the PSEN with red LED - it shows you that you need trace the problem. In theory, usage of such scheme with ONCE TM mode together, may help to locate a step where the problem is placed. Another issue: latch P0 via ALE and P2 with PSEN with immediate MCU reset state after PSEN is activated, may show you which address was accessed due malfunction. Then it is possible to scan program binary for that address and try to find a problem.

Regards,
Oleg