These are very, very typical light leaks - actually, I think 99% of 35mm SLR light leaks look like that - and they are due to light leaking around the camera back/door. You say they only appear in about 2.3 of your shots - again, very typical: they only appear when the light is a) strong enough and b) hits the camera at the right angle. Mostly, I would concern myself with the hinge area next to the take-up spool (but honestly, why bother when replacing all of it is so simple!). I can assure you, I have personally had light leaks that had no apparent cause after a visual inspection of camera, yet went away with a change of foam - also, you will really notice various permanently compressed sections, sections that are eroded in out-of-sight areas, etc, during the removal of the old foam. I don't have much money, but I would bet most of it on the fact that these will be completely cured with some careful application of new foam. Get the interslice kit, set aside an hour or so, and say good-bye to these forever.
Very nice shots, too!