I'd start with checking for loose wiring on the opto, maybe bang with your palm near the area and see it if can make it happen without using other switches (flippers) as it is a bit of a pain to get to. Should eliminate switch matrix. it could be also be a opto mount breaking the beam, I had a similar promlem on the right opto with loose wiring another real pain to get to 
Hey KTM yea it seems most likely to be a vibration based problem I think,
your right the Optos look like a Pain to get to, upside down on the ball guides
ive been wanting to give the Pf upper a full strip and clean and rerubber,and sort any little issues like this out,
but cant bring myself to put the machine out of action just too much fun to play!
Peter...
