I had a problem with some kodachrome received recently too. Some slides showed a very distinct part(like 1 stop underexposed) across some of the slide.

here's an example. I was using a fill flash, and I know that the shutter and sync is working properly, and 80% of the others don't show this problem. Does Kodachrome have to be re-exposed in the processing?

IDK if this could have caused the problem.

please forgive the dollar signs. my scanner software is a demo !!!