Quoted from sparky672:Let's say the outhole switch is telling the MPU that a ball went down the drain... then the playfield resets and play continues, UNTIL ball 3 goes down the drain, then the game is over. In other words, carefully check the outhole switch to ensure that it's not being activated by normal vibrations.
https://www.pinwiki.com/wiki/index.php/Gottlieb_System_1#Testing_Switches_in_the_Switch_Matrix
My guess is one of the playfield switches has a bad diode and is causing a phantom switch closure for the outhole.
Too bad Gottlieb documentation isn't freely available. I'd look at the switch matrix and maybe point you in the right direction.