A couple things that may be part of your EEPROM corruption problem:

- Do you provide protection against load dumps (24-50V+) and other
  transients?
- Does your circuit run properly over the normal operating range of
  the battery (generally 9-16V). Do you have any built-in detection
  of under/over battery voltage? Does the code enter a safe mode
  when this condition arises?
- Are the lines going to your serial EEPROM short and guard banded?
- Do you keep checksums for both the EEPROM data and the data that
  is to be written?

I've had problems with EEPROM corruption before. It can generally
be traced back to either noise on the serial lines or the MCU
getting "lost", especially during normal EEPROM access.

Jeff