Originally posted by Ex Finn.
Sounds like a sticky aperture mechanism. You are the first that I have heard of.
Agree. Quick test is to dismount the lens look at it from the rear. Move the aperture lever so that the aperture blades are fully opened, now let go. The aperture blades should snap back closed. No hesitation, no slow movement, just spring loaded SNAP. If there is any hesitation then that is the problem. If not, then I'm stumped, you seem to have tested the other possibilities.
Check:
Metering is not set to spot
Exposure compensation is set to zero