This is not my video, but is EXACTLY what my game is doing. » YouTube video
Basically, earn multiball by shooting the lower playfield and it immediately starts repeating a horrible tune. Then drain lower playfield balls and neither will activate the VUK and the game just gets stuck there.
My machine is running Ni-Wumpf CPU and Driver boards as well as a Great Plains Power Supply board. Game plays 100% correctly until multiball, including the VUK working and the lower playfield working as it should.
I'd have noticed this problem a long time ago if it didn't take me 100 games to get multiball just once.