Check for broken solder joints on sound board 10J3 (next to dips) and CPU 1J8 (UR corner).

Re-punch all wires in 10P3 and 1P8. If that solves your problem, RIP AND REPLACE 10P3 and 1P8. (Re-punching is a temporary fix!)

_{Post edited by TheKorn : J's are for pins, Ps are for plugs.}

Check for bad IC sockets on the speech board.Did you see a little "scanbe"on the sockets when you changed the roms?If so you need to replace the sockets,also the data link cable between the main sound board and the speech board could be gving you problems.My blackout has scanbe sockets.There should be one empty ROM socket on the speech board,have a close look and see if you can see a very tiny "scanbe" on the socket.If so you need to replace them.It's a miracle they have worked this long.

Admittedly I'm being a little pedantic here, but that doesn't really guarantee anything. Cracked solder joints can be very difficult to spot with the naked eye. Much easier to spot with medium magnification, and a dead giveaway is when you heat the joint and an outer ring liquifies while the center doesn't.

(...but you're *probably* right that they're *probably* fine if the board is in very very good shape otherwise. Just not guaranteed without looking VERY hard at the joints.)

Another way to think about cracked solder joints is; that if you already have the solder gun out for some other repair (caps, sockets, diodes), it only takes a second to hit those connector pins again...