I don't know if this will help or not, but my 500c/m was acting really weird. I'd press the shutter button and nothing would happen. When I let go of the shutter release the lens shutter would stop down and take a picture. So everything was after the fact. It would do that with my 120mm lens but would work perfectly with the 60mm. It was very, very unreliable.
Originally Posted by Kevin Caulfield
Turns out there was something wrong with the camera body itself and it needed to be repaired. I broke a spring when trying to "fix" it. The camera was made in 1974 and I don't know if it was ever cleaned and lubed since then. I know I never did it and I've had the camera since 1994. So you may want to bring it into a repair place just to have that done, especially if you have a C/M model.