Rubbers: I don't know what rubbers I have. They're black. They were on the machine when I got it. I'm guessing they're the Stern OEM, given the young age of the game. It had 2 previous owners, so I can't say for sure.
Coil power: Already moved it down to the lowest setting. I didn't see any difference between the 3 settings.
Slingshots: Capital idea. I put some fatty thick washers under 2 of the star posts for each sling (no point in shimming the post that's not involved in the slingshot action). Played a couple of games. No airballs--I'll keep an eye on it. During gameplay, one of the left sling's leaf switches slipped out & was in front of the rubber...which reminds me that those slingshot leaf switches (on both sides, I think) have snuck out a few times. Weird.
Flippers: Another capital idea. Raised the flipper bands a little. I think they scooched back down after a few plays. Perhaps I'll instead loosen the bat shafts, shim them up a tad high, & then reclamp them.
Wax: Probably a good idea. It's super clean & already too fast for my meager skills, but a waxing is always a good idea.
Other: Come to think of it, I get many other bounces pretty frequently. I watched the Captain America pop bumpers in action more closely last night, & the ball definitely hops around between them, but it never got out or over into another part of the PF. Also, on a good plunge, sometimes the ball falls out of the shooter lane above the rollover lanes & then hops right over them & into the pops, skipping the rollover lane switches altogether.
-J