The contents of the 5101 RAM does not disappear instantly. A lot of the data will be the same even after 15min disconnected from the battery. WMS uses some kind of RAM checksum so it usually detects memory problems, but if only a couple bits flip that not involved in the RAM check, it may not trigger a restore factory settings.
The power down and power up sequence is critical time for memory integrity. Make sure the power on reset circuit goes HIGH to LOW before caps drain off at power off and LOW to HIGH at power on. If a transistor breaks off in the reset section you can go Z state to high or if the capacitor goes bad the reset pulse is too short.
Check for about 4.5v at pin 22 of the ram chip with power off.
