Replace all of the Cherry DA3 microswitches with new ones. They are pretty much always bad if they have never been replaced.
Micro switch body
http://www.pinballlife.com/index.php?p=product&id=1860
or you can buy the whole switch with the wire on it which I recommend.
Check your manual for the correct part number.
Don't have a manual? That would be the first part I would purchase to keep you in business.