Could be as basic as a connector... machine is left on for a while, connectors have tarnished over time, heat builds up causing more resistance and tarnish - signal drops out.

The IC17 and 18 were these the non inverting buffer chips on the schematic that turned to to be inverting on the actual game?

Thats OK - was  a misprint, doesnt really matter if they were inverting or not, its the inconsistences of the input/ output thats important...

.. schematics are fine, but its always best to have the game in front of you, anything else is just a guide