Lon,
If I understand correctly, the game functions fine all the way to the end where you max out your score like you usually do, but when you go to enter the High Score - it crashes..
I believe that it may be the 5114 cmos static ram at location 1C. When they go bad, they cause all kinds of errors in your game. Keep in mind that this is where your game settings and High Scores are stored and kept going by your battery. If you haven't done the lithium battery mod, you should do it to prevent any acid damage to the MPU and it will rule out any potential power problems from the 35yr old battery holder. Next I would desolder the 5114 at location 1C and replace it with a dual wipe socket. Install a new 5114 cmos in the new socket. Reinstall MPU board, boot game, restore factory settings, reboot game, go to service menu, make some change (ex: freeplay: yes) and then save it to the cmos. Reboot the game and see if everything is working okay now..
..There are also the 1-chip solutions to replace the battery AND cmos once you have socketed location 1C - either way should fix the problem.