Quoted from SaneWilly:That sounds like a leaf switch triggering. Go to switch test and hit the flippers and see if some other switch is occasionally triggering. A few of us have had to adjust the upper playfield right leaf switch for a very similar issue.
I had to do this. The right leaf switch was firing with the flippers and sometimes I think the shaker. Then later a couple of other leaf switches on the upper playfield were doing the same.