Just in ordinary play mode, carefully exercise each instrument button. DO ANY of them double-stroke on you? What about the posssibilty that when you press the BD button, for some reason one of the nearby button contacts is also activating? As in perhaps the circuit board is flexing.
Take it apart and clean the contact surfaces and make sure all the mounting screws are present and no corners are snapped off the board. Look at the rubber membrane around the BD contact, is it in good shape? Flex the rubber sheet while it is out to look for little tears in the rubber around it.