Whew...what a long and frustrating night.
I decided to rebuild the lower 3 flippers with new rebuild kits from PBL and new bushings. Probably overkill, but we have a lot of plays and we've noticed degradation so, I figured why not.
To my chagrin, the flippers actually played WORSE after the rebuild. I could not backhand the left ramp, I couldn't make orbits, something was very wrong.
It took me way too long to diagnose but to hopefully save someone else time: the coil sleeves in the PBL kits were too short!
The correct sleeves are long enough to insulate the plunger as it passes through the metal ring. These sleeves were not and the plunger was grabbing on it and getting friction.
Once I replaced the sleeves with the right size the game now plays like new as I originally expected.