Sounds like a transistor problem. You might have a bad transistor on the driver board. If you haven't already, take a trip over to pinwiki for some info regarding the driver board transistors. You'll need a multimeter to test them.

Alternatively, it could be as simple as a bad connection or ground somewhere as well. Although since your post suggests that everything works as it should (excluding the ball return), I don't think it could be that. Still, this generation of Gottlieb games are notorious for poor connectors and even poorer grounds.

http://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Driver_Board