[adan]

Likely the foam rubber seal around the window in the back that shows you the film cassette to identify which film is loaded is getting stale and corroded, and allowing light to leak in.

All that black foam, (the stuff the mirror hits when it swings up, also) tends to turn into black goo after about 15-20 years. Fairly common with aging R4/R5s.

You may even be able to replace the foam seal in the back yourself, if that is what it turns out to be. A simple peel-and-stick job once you've removed the old foam.

The reason it is intermittent and varies in brightness is that it is a very faint leak, and will vary with how strong the light is hitting the leak point, and how long you left the film in one place between exposures.

Crazy-making - but easier to solve than some camera problems.

(The red coloring is also because it is a slow leak - red light always seems to penetrate a bit further, noticeable in the red/orange/yellow smear you get on slide film right where the exposed leader transitions to the film that was protected by the cassette during loading.)