Since you've already replaced some of the springs to not much effect, your drops sticking on the way down sounds like it's due to too much friction somewhere - often the rounded point of the metal arms that go through the plastic body of the drops and slide along the switch blades to activate them are being pressed too tightly by the metal switches, usually because the metal blade of the switch is bent too much toward the metal arm and pressing against it too tightly. Those switch blades should be clean too - there's often some sticky stuff on the metal switch blade that the end of the arm is trying to slide against. I know High Hand has a bunch of drop target banks, but maybe you can get in there with Q-tips and alcohol and try to clean the black gunk that seems to accumulate on the metal switch blades. Then check to see how tightly they are pressing against the metal drop target arms and bend them away gently (from the bottom) if they're pushing too hard against the arms. If you do that, make sure they still open and close as they should afterwards.
As for the drops not resetting consistently when the solenoid fires, again, make sure there isn't extra friction in the reset solenoid mechanism, and that the sleeve and slug are in good shape (the slug is clean with no mushrooming on the end, and sleeves are cheap - replace them if you're not sure). If that all looks good, Gottlieb gave you a simple way to move the solenoid slightly one direction or another - usually 4 screws that hold the solenoid bracket in place are in slotted holes. Over time, the bar that presses onto the underside of those same metal arms I mentioned earlier (and lifts all the drops together), can cut a nice little divot into those arms, as leckmeck mentioned, so in effect, you don't get as much lift when the solenoid fires - and the divots aren't usually the same depth from arm to arm. You want to make sure that the solenoid mechanism is free of binding/friction first, then if so, you can move the solenoid a little to give yourself a little more throw. See if that helps.
It might be possible that the solenoid isn't firing as strongly as it should too, either from a bad solder joint somewhere or the coil itself partially shorting or something. That's probably the least likely explanation, though.