Ok,
- I have done what was suggested: used about 1/4 can of throttle body cleaner to clean it. Twice. No change.
- tried adjusting the throttle linkage cam nut (11mm). No change.
- After a great deal of tinkering and trying to figure out what does what, I am almost positive that the butterfly valve is getting stuck on the intake throat, possibly because it's being allow to rotate too far, it's off-center, or something. It doesn't appear to my untrained eye to be off-center, so I can only assume it's just rotating too far and getting stuck on the wall of the intake throat.