I don't think batteries have much to do with it...
I know that if I was an engineer, I would put a transistor to keep voltage the same no matter what the battery's power is.
On the other hand, I know some cameras use a condenser hooked up to the timer. Condensers tend to dry our and go bad with age. If the camera goes back to the 60's or 70's, it's very probable this is the problem.