I have my electronic safe working around the 628A and it works like a song. Out in the field for years now. Never had a problem with eeprom corruption. I save the user codes in EEPROM and read them from there for comparison.

I suspect your code is 'running away' either due to power glitches or maybe an oversight in coding. To confirm this theory, I can suggest you to try MPSIM(I use it extensively to debug). Put a breakpoint on every WRITE statement which puts data into the EEPROM. This you can find by getting the addresses from the LST file. Then just run your code. If it breaks, you have the answer.

Power line glitches may be hard to catch. In my case, the safe works off 4 D size cells (6V in all) directly. I know I am violating the Vcc(typ) spec. However, what I'm trying to convey is to power the unit by batteries if you suspect power line glitches.

Hope you are able to 'catch the culprit'.