Those are 6-volt lamps. You won't find 120 volts on those no matter how you check.
If the weak socket doesn't clean up with a dremmel, steel wool, sandpaper, or whatever, just replace it.
Do that with the power off.
It would be very unusual if the problem isn't the socket itself.

Also, not real important for this discussion, but it sounds like you are discussing controlled lamps, not general illumination (always on) that your post seems to be about when you say "regular playfield lights".