Have you monitored the 5v rail on the board while testing? i imagine it's raising up/down too.....
I'd be looking around the onboard regulator oand/or any capacitors and diodes associated with mainly the 5v supply.

I can't see how digital chips have a varying LEVEL of logic, so i'm thinking it's more to do with supply voltage and not logic/chips being faulty.

my 2c ;-)

checked all that. constant 5.01 volts
Plus I did the old freeze spray test. freeze spray on I.C. voltage dropped, finger on I.C to warm back up to room temp voltage rose. Replaced I.C. voltage was constant 5 volts & intermittent fault was gone. Before I.C. was changed door switch would close and open for a few minutes on its own then it remain closed even when door was closed (even by disconnected switch).
Replaced I.C. fault was gone even after an hour of running.
Some one else tried to fix this fault by changing the lm339 that relates to the coin door switch and start button.( before pintoxicated owned it)