It thinks it has an extra ball. Go into switch test and check both of the 24" optos. Make sure they change state. If the right switch sees the ball, when the ball is going from left to right it will drop the ball half way( sdtm).

There can be a ton of reasons why this happens. Doing the mist ball test will sometimes tell you whats up. It could be the spriraled wire has a bad connection, opto weakness, or in many cases the board itself. There is small inductor/choke that can fall off the board due to age and vibration. Do yourself a favor and replace the mist opto board with a new one from homepin. You will at least get new components and a status light visible from the coin door.

Had the same issue. The 'phone cord' cable had a break in it..and once it hit a certain point of travel..it would lose connectivity and turn the magnet off

While I agree with the home pin board. Do a little troubleshooting before you drop $80 for a new board.

I'm going to beat terryb to it this time: http://pinballrehab.com/1-articles/solid-state-repair/game-specific-toys/200-mist-multiball

I always suggest repairing original boards over replacing with aftermarket. Check the choke and look for dry joints on the connectors. Check all wiring and perhaps replace the opto TX & RX sensors.

After that I would replace the board, there comes a point when the time spent trying to fix something that critical parts are no longer available for is not worth the effort and it's prudent to replace.

Available here:

USA:
http://www.pinballlife.com/index.php?p=product&id=4362

Australia:
http://www.johns-arcade.com/#!product-page/c1cs/59919533-df8f-8f79-cd42-49f29c39302d