I think it is more likely the external I/O and input/outputs pins that may go faulty rather than the inner core.

Write some test code that execises all the ports and reads (and writes) at every bank address, with a known good micro, then replace with the suspect micro. Break the code down it to the simplest form... for each of the I/O functions.... Too much complicated code will obsure the fault.

I recall once we had a faulty pager transmitter overseas (China) and we speculated to the customer what the fault could be, for a week or so. The sympton looked like a faulty micro... but it ended up being... being a faulty logic gate, when the product was returned to us.

We later discussed how we could of send the test EEPROM that execised the CPU and pin-pointed the fault to the I/O and then the logic gate... and save a week of time.

Joe