(found my last post on this, reposting here...)

The opto boards that let the game know that it has balls stacking up right at the ball trough where it shoots up to the shooter lane is the usual cause of this.

Your manual is here:

https://www.ipdb.org/files/4492/Stern_2000_Sharkey_s_Shootout_Game_Manual_with_paginated_schematics.pdf

PDF page 79 has the trough assembly information.

The optos can get weak, but it's mostly broken solder connections, broken LED's.

These are part number 520-5173-00 (Transmitter) and 520-5174-00 (Receiver), if you just want to buy replacement boards.

These are testable in switch test, and in ball clear test, so go into switch test and block the optos, see if that's the issue.

Your ball trough black microswitches can also have issues, though I probably fix 20 bad opto boards to one black microswitch issue. Sometimes there is trash that prevents the switch from clicking, sometimes the balls can rest on a small 'divot' and not roll properly onto the microswitches. Once in a while, you'll need to bend the switch actuator to get it to reliably activate when a ball rests on it. Since you are there, make sure all that is working.

Your ball clear diagnostic test shows the status of all the switches here.

On any machine this age, I'll pull the opto boards and rebuild them with fresh optos.

I use part #LTL2F3VEKNT and I buy from Mouser:

https://www.mouser.com/ProductDetail/Lite-On/LTL2F3VEKNT?qs=Sv%252BigzQKaoVYLcEZH8BKGw%3D%3D

Replace both the transmitter and the receiver LED's at the same time with the same part. Both the transmitter, and the receiver LED's are the same part number, LTL2F3VEKNT. Don't do half the job. These LED's are quite a bit brighter, and I've used them on all visible opto boards with no problems for years and years.

While I have the boards out, I make sure the optos are straight (aiming the right direction), and I resolder the connectors. Just pulling off the connector and replacing it might fix the game 'for now', but won't be a permanent fix.

Let us know what you find!

From the game's perspective:

Sometimes when the ball is popped upwards, it falls back in. It rests on the next ball. Now I've got two balls stacked on top of each other, and no balls at the shooter lane.

So, the game has a 'stacked ball' sensor, for just that possibility. If it's somehow gotten a ball stacked up, it first kicks the ball popper to pop the top ball out.

Now the ball is in the shooter lane. The game doesn't want multiple balls in the shooter lane, so when it sees a ball (by activating the shooter lane switch), it activates the ball launcher which kicks the ball into the game, so that ultimately the ball can end up back in the trough in proper line-up.

If it didn't clear the shooter lane when a ball is waiting there, it might (due to malfunction) end up with two balls, three balls, four balls waiting, and then the shooter might have trouble kicking all those balls... so as soon as one ball is seen in the shooter lane, the game shoots it into play.

The problem is when that top sensor is disrupted. If the transmitter doesn't transmit, or the receiver doesn't receive, the 'seeing eye beam' is disrupted. It only sees a clear unblocked ball trough when the transmitter is transmitting and the receiver is receiving. Any disruption, and the beam isn't sensed, so the game now believes that a ball is stacked up.

Even when a ball ISN'T stacked up, the game now thinks it needs to pop up a ball to clear the stacked up ball. So it pops a ball out, then the autolaunch switch puts it into play.