you did not mention the game but is this Gottlieb Oklahoma? you really need to make that clear...
the score motor controls this. i don't have the schematic in front of me but usually it's at 4a. One of the big contact switches handles this. Make sure that's clean and gap is correct (too far or too close will cause the problem you mention.)
but also check the player unit itself. Sometimes the coil stop breaks off inside the coil and can cause inconsistent "pulls" on the player unit coil. or the player unit plunger travel has been some how changed in other ways.