Thank you all! I tried to assess the state of the light-insulating foam on the camera and... to be honest; there really isn't any foam left... some glue, and what looks like it might've been foam at one point, but that's about it. Thinking about it some more; the timing at which I shot the film was something like: Put it in the camera, leave it there for a couple of hours (which could explain the start of the problem at the start of the roll), then I shot a bunch of pictures fairly quickly (middle of the roll), and then left it in the camera for an entire day before shooting the remaining 4-5 frames (which could explain the problem at the other end of the roll). I might be grasping at straws, but I think the timing might make sense *scratches head*