Oil on the aperture blades.
With all the glass elements removed from the barrel, I was able to flush the aperture mechanism. In this case acetone was used. but be careful. If you get acetone on the outside of the barrel it can take off the numbers. Also, acetone can remove black paint that may be used internally to prevent flare.

Without taking the entire mechanism out and soaking it, it can be difficult to get the blades clean. New oil keeps seeping out from the perimeter onto the blades. With some persistence I got the blades all clean without having to take the lens helicoid apart to get the aperture mechanism out.

Bad news is that the aperture still would stick and not close down all the way.