Trouble is not only on the digital side
My old analog camera is also giving me trouble. The symptom was that many pictures had just a small fraction of the total field. This can happen when shooting with flash if the sync speed is not correct (flash fires when shutter is half-open only). However this time I was shooting outdoors without a flash.
After a few iterations with Google led my to a photo on Wikipedia with exactly the same type of residue I've found on my camera. A few more searches led me this article that explains the causes of the problem and a possible fix.
Aging and heat turn a piece of foam inside the camera guts into an oily tar that ruins the shutter operation. Did you think you can keep on using your old camera, maybe not if it as a twenty years old Canon EOS.