from your description, it sounds like it is the 80mm lens...
...i have the same camera and have not encountered that problem - pure dumb luck, my friend at work.

did you repeat your trouble shooting steps with each battery? perhaps the rebuilt battery is the root of the problem...the part about it working when manually stopping down and firing is baffling.

wish i had more helpful input...

breathe, relax and enjoy.
laugh more, smile more...i do whenever i get near a mirror.
smiling gecko, aka kenneth