I had a similar problem with mine- it wound up that it was simply slightly out of alignment. I found that I was resting my hand on the playfeild making it move as I removed hand.
Try slightly smacking the playfeild while watching the switch matrix. You may have to be a little aggressive. Mine would flash on and off while smacking. I finally adjusted it by adding a paper shime to the left side opto. May/may not be it- but easy to test.