that happened with mine when it was relatively still new. I troubleshooted a few things and then the problem went away, not really sure what the issue was - wish I had an answer for your, but Im going to follow this thread to see what the solution is.

LOL, I had a similar issue and can't for the life of me remember what I did to fix it as it was a while back, other than I think it was a loose connector under the playfied that was making intermittent contact.

Had similar issue with my AC/DC. Try re-seating the plugs to the ball trough...

reflow the solder on the optics

also check if the switch in the trough is registering when the first ball hits it. use the ball not your finger to test the switch.

ive had this issue before and one or both of these two things fixed it

The IDC connectors on each side of the ball trough are notorious for coming loose. One operator told me he zip ties those connectors in place. Could be an intermittent connection caused by vibration when the first ball is kicked out?

Not that this helps, but did see the Mustang pin owned by Ford themselves at an autoshow was launching two balls at the same time as well.

More than likely the Plugs on your ball trough optos. Especially if you have a shaker. With the game off, unplug and plug them back in. One on front board of the ball trough, one on the back of the ball trough. You can get to them easily through the coin door.

ST is notorious for these plugs coming loose. Or intermittant contact. So much that the design was changed after ST on all future titles.

If that doesn't fix it go into switch test and test the trough switches to see if an opto is out.

Yeah I had to reflow the solder of the ball trough opto on my NIB Mustang LE. I think it was the one that detected the ball just before kicking it out.

I had that on Avengers & a wire was broken under the shooter lane switch.

That will do it. I've found all kinds of junk in the ball trough. Bulbs, screws, drop target chunks, etc.

I hate it when you have the playfield tipped up and you drop a screw and it goes to the back of the playfield and you can't find it.