Can't believe this has been going on since April last year.
I eventually got this looked at but still couldn't pinpoint the problem. Possibly a switch diode problem thought to be the cause.
Contacted American Pinball and they said it was most likely a problem with the opto interface board.
Ordered one on Boxing day and it turned up the following Wednesday - pretty good I thought, even if postage was $67 nearly double the cost of the board itself!
Fitted the new board (get me, genius

) but it didn't solve the problem
Spoke to
AP again and they refunded me fully which was very decent of them and suggested the following;
"Hey Paul,
Sorry that opto interface board didn't solve the issue. As I said, it could either be this opto interface board or something with the SW-16 switch boards.
Try flip-flopping the 2 switch boards in question.
When you change them, you'll need to also flip-flop the DIP switches on the board.
If the problem is isolated to these boards, that would mean Right lock 1 and Right pop bumper will now show the exact same behavior as your ramp entry and front of stage did.
If the ramp entry and front of stage switch behavior stays the same after the flip-flop of boards, then this is either a code issue or issue with the P-Roc."
I haven't tried this yet as quite frankly not sure what or where exactly I'm supposed to be looking (from genius to thicko in one move

)
Any hepl greatly appreciated
Cheers