I send my colour film to a lab to develop. Recently I've been getting uneven areas of development, or horizontal 'streaks' - I'm not sure what to call them. I've attached an enlarged detail, as examples. (I've upped the contrast to exaggerate the effect).

(I don't know whether it's connected, but on a previous roll of film, some of the negatives a tiny areas of hard congealed shiny almost 'varnish' type of substance on the edge of the film.)

At first I thought this was camera scratches, but this doesn't happen with every roll of film. At the same time, could this be caused by something in the camera?