That sounds like something going on with that camera. I wonder if it's irregular in releasing the shutter, maybe it doesn't and causes black frames, or fired accidently when you happened to be holding it toward a window/light source and you wouldn't have realized at the time it released and exposed a frame.
Maybe do a test roll for each camera you've been using; keep track of what camera, what film, what you photographed on each frame. It could be it's not releasing the shutter when it should, releasing it when it shouldn't, not moving smoothly/properly, etc. Seems like it might be advancing film properly but not always firing the shutter to expose the film for each shot.