I had a problem similar to what you are describing - it was the wires going to the switch in the glove - the wires had broken over time due to the movement of the hand and it would cause an intermittent connection. You might want to check the wires behind the playfield/glove in addition to what EV mentions above.

look at the wiring harness and test the continuity of the wires (and move them around when doing so) - the problem for me was intermittent and it turned out to be a broken wire in the harness (sometimes it would make contact depending on the glove movement/location, etc).

More food for thought .