I replaced the PC motherboard battery and the PRISM card battery.

Upon cold starting the machine it booted into the updated 1.5 game code right away.

After power cycling the game it displayed the same “Bad update game code checksum” that is was showing prior to battery replacement.

After several more power cycles I saw the 1.5 code one other time, but nearly every single time the 0.80 code is what boots.