Problem corrected! I am not positive of which of my changes made the difference.
I took the tensioner apart again and recleaned everything.
I put a small pad of felt under the thread holder.
Most likely though, was that I discovered the screw that holds the presser foot onto the machine was very loose. Part of that mechanism also pushes against a spring that (I'm guessing-couldn't actually see it) pushes and releases tension on the thread tensioner. With the screw tightened, there was more force on that spring.
In any case, I had two hours of lovely, frustration-free sewing and the thread tension feels right.