I have a Spotmatic that feels as though I am stripping the gears when I advance. Even without film in it, it is the same. It's not debris because I flushed everything out with lighter fluid until completely clear with the top and bottom off the camera.
There are many variables here with this sort of thing: curtain apron fault (even twisted strings!), loose gears, corrosion where the curtain slides causing friction, even too much curtain tension.
Momus, have you felt the tension on those curtains? Advance half way so that the metal strips at the ends of each curtain are in the middle of the film aperture gate. Then, carefully lift the mirror and tape it's underside to the top of the lens mount area so that it stays up (maybe this Canon has a mirror lock so that that will not be necessary). Now, with the camera back facing you, carefully and gently with your fingernail, move the curtains one by one (ie, move the second curtain's metal strip toward the right and the first curtain's metal strip move toward the left. That will open the curtain area as if for exposure. The level springiness will indicate how much tension there is on those curtains. If too much (they should have only light to moderate tension) you need to turn the tension screws to lessen the springiness. On the Canon mechanicals it is a bit harder to do as those curtain screws guiding curtain tension have small levers attached to each and must first be loosened in order to remove the end of each lever from the ratchet which they employ in order to prevent slippage. It's rather difficult to describe in words but with the Spotmatic all one need do is put a drop of lighter fluid on the two set screws which hold those curtain tension screws tight and then you can freely adjust that curtain tension. - David Lyga