Bottom of the frame, so likely a leak at the top of the camera. The edge of the film is not affected, so it happens when the film is behind the gate, so it happens around exposure time. This suggests something at the front side of the camera. It's a narrow line, so the light is relatively collimated.
You could try placing an affected negative back in the camera so you can see if the light leak aligns with a structure in the camera. The usual trick with the flashlight in a dark room also applies; shine a powerful flashlight into the camera and observe the outside of the camera to see if you can find any place where the light gets through. You'll have to look at all angles given how directional it is.
The reason why it's intermittent is probably due to the usual factors of the time the film spends in that place (i.e. time between shots), lighting conditions and handling of the camera (held up to the sun etc.)