As usual, check the trough switches. These may be bad even if they look good, and have been cleaned.

For years, Williams switches were assembled backwards - instead of contact to contact, they were contact to rivet. Not a problem in EM games, but a BIG problem in aging early SS games with gold contacts.

This is very easy to see if you look at the switch. If those trough switches are the original bad ones (and they likely are) it is very common that you'll have this issue. The effect of these backwards switches can be felt particularly bad in places where the balls sit - mainly the outhole troughs and the lock holes.

You can disassemble to switch and turn the rivet one around, or just replace the switches with new ones.