I,ve seen the magnitism problem once before, but on an outhole (rocky). The plunger would stick. Took me hours to work it out.

The problem was that the previous "tech", had replaced the coil bracket with a bracket from an EM. So every time it fired, it held the plunger in. The smarty "tech" screwed a wood screw into the underside of the playfield so the plunger did not travel all the way to the coil stop - because it would then stick.

Using a coil bracket from an EM (AC) in a SS (DC) will cause this problem of the plunger sticking.