The long range opto board.
It only takes a few minutes to remove and reflow. definitely an easy place to start.
My personal guess is your problem has to do with the long range opto beam. It could be the sender, receiver, wiring, or board. When "mist" mulitball is running it looks at the long beam to see if the ball was knocked off. When the ball IS knocked off the magnet the switch will be "closed" with nothing blocking it (the receiver sees the transmitter beam).
The following scenarios are seen as the same to the CPU and the program will assume that the ball is blocking it.
1. The long opto works and is blocked by a ball.
2. The long opto- sender, receiver, opto board, or wiring, are faulty.
The program assumes the last ball is in the mist gates and ends your play.
Mine would run the "mist test" perfectly every time- but during a game the vibration from other balls and flippers would cause the problems. I could slightly press or shake the playfield and get the opto the open(as if something were blocking the beam.) I used the "switch test" (not mist test) with no ball in the path of the long opto and ran my hand back and forth to test the switch. try shaking the playfield. With nothing in the way the opto should always stay closed.