I have seen some problems caused by weak batteries. I think that machine
uses the TimeKeeper series from Dallas, which means you have to
carefully take the top off (do a search for the process) and replace the
original battery with a new one.
Otherwise I don't have any great ideas, other than running the self
tests to see that all switches/optos respond as they should.
One other thing to consider is it may be the optos are dirty, take a
clean Q-tip and rub it in the gap (dry tip). If the machine is in a
smokers home then you may have to clean the optos with lens cleaner for
cameras.