I have a thought that the problem with your Hexar is the exposure compensation switch.

For 20 years that switch was probably never moved and sat on +0.

Over time the contact surfaces of those switches build up a hazy coating. This means when you moved it to -.3 or -.7 you lost contact and the camera didn't know what it was supposed to do. Therefore you got massively over-exposed frames.

The easiest solution is to leave the selector at +0. Next you can clean it off a bit by moving the selector all the way back and forth about 30 or 40 times. Of course the best is to have the switch opened and electrical contact cleaner applied.

Try another roll and make note of which frames were set at +0 and which were offset. This may prove the theory.