This is known as an AS relay and is a remarkably versatile and tiny stepper that can be really fiddly to adjust properly. Here are Gottlieb's instructions:

https://www.ipdb.org/files/2233/Gottlieb_1975_Soccer_AS_Relay_Adjustment_A_11492.pdf

You may also find other topics here about this device with a little searching.

I would start by cleaning the wiper board and contacts to see if that helps. Like any other stepper you should be able to pull the armature all the way down, let it go, and have the stepper advance cleanly to the next position. If yours isn't doing that have a close look and see if it's stepping too far (more than a full step) or if it's rebounding and stepping back a bit (or less than a full step). That would help determine what adjustment to make.

Beware that this isn't a device that responds well to a shotgun approach. I wouldn't adjust any more screws until you have a better idea of why it's not precise because the adjustments for back sliding and overstepping are different. Once you know which way to go only loosen screws just enough to get the adjustment you want, make a tiny change and then snug them up again and test to see if things are working better. It may take a few tries to get it dialed in.

/Mark

Great advice Mark G, I would also add that even with all the proper adjustments there can still be issues with a sloppy match unit. This can occur when there is too much play between the nylon Center spindle (labeled B in the diagram above), that attaches to the ratchet and. Snowshoe (labeled a in the diagram above.) 00-90 units that may have been abused, or have seen heavy use, could exhibit this behavior. It is tricky to repair. Check to see how much play there is between that nylon Center spindle and snowshoe arm. I have had some success with bending out the tabs on that arm that engage the spindle also, adding some Thin metal shims is it another option.