EM's always seem to need tweaking time to time, regardless of brand. I have a dozen running and my worst offender has been a 70's Bally but have a Gottlieb Domino that currently needs three issues fixed. Gottliebs in general have short contact arms and seem to be very touchy in regards to contact point clearances. The other three major brands are more forgiving there.

Winners for highest reliability?

Williams Heat Wave which has not a single issue in over a year and Chicago Coin Moon Shot.

Both have been anvils since setup.

Keep in mind that these machines are incredibility complex electro-mechanical devices that are pushing 50 years of service. Not all were treated well, stored well nor serviced by the best people at times.

Your mileage will vary.

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Sooner or later anything goes out or they would not require refurbishment in the first place

Worst EM I had to work on was a Gottlieb Jungle King, spent weeks getting it to run right and would only do so for a week or two at a stretch.

That one machine made me swear off any further Gottlieb EM's, Domino is my last one.

Yes, usually it is easy fixing.