The fix: I pulled the slide cover off, being careful to lock the spring through the end plate with a drill bit. I then carefully unwound the spring by holding the tube. I noticed several times it would hesitate, then roll again. I suspected a broken spring.
Once spring had lost all tension, I took out the four bolts that hold it to the slide. The end plates come off easily. One end just holds the end of the tube in place, the other end, with the gear, is attached to the spring.
The spring is in good shape. I noticed the end plates each have tube about 10 inches long that slides into the roller. They felt rough, had a deposit of black stuff on them. They fit into a plastic bearing at each end. It appears over time, dirt and weather have caused the roughness.
I cleaned the end plate tubes with a wire brush and coated them with white grease.
Reassembly: I decided to do everything with the slide in. I loaded up the spring with eight full turns, locked it in place with drill bit, then installed the cover. Then, carefully, I held the roller and let it retract the cover. This told me I have enough tension with eight turns, to retract the cover completely.
The final test was to go inside and extend the slide to see if it dragged down and barely moved. Surprise, it extended and retracted with no hesitation. So the issue was the slide cover was holding the top of the slide, causing a bind. Cleaning and lubricating the end plate tubes solved the issue.