On page 65 of the manual (from ipdb.org) - The right slingshot and middle drop target #3 (and lower level #5 target) on on the same row. The other switches in common are the tilt, 2 X bonus on lower level and the top target. So I would confirm all these switches are out and then I think the problem would be the entire row. That row wire is PIN 14 on J2 of the MPU. I'd check if there is any corrosion on the pins of J2. Other than that, check that there is no broken wire (brown with a white trace) to any of those switches.