Actually managed to get it going last night - replaced another two optos with some more  jaycar ones i had lying around, still had similar issues, so reflowed every single joint, tested all continuities, and gave the interboard connector pins a sand - these seemed to sit too easily in one place, so i wedged the two boards apart another mm or so, where the pins were a bit more snug, and am thinking this may have been the issue all along - if the pins broke contact breifly, the opto might flick on and off?

Anyway, worked for 5 games straight, so will see if it lasts.