Thanks for all the great advise, Success at last  . The  problem occurred after re-ruberring the machine. New replacement rubber behind the drop targets had pulled the contacts together.

Machine now scoring correctly & pop bumpers seem to be ok? I dont know how the switch behind the drop targets could cause pop bumper probs? maybe a coincidence. 

By firmly bumping the playfield i was advancing the score & when activating drop targets manually, i was getting unusual score advances of 200, 500 & 700 for different targets. Thats what led me to that switch.

Something so simple caused so much chaos!  Here's to a great team of problem solvers.