I don't change film speed. I always shoot box speed, though now that I am home processing, I have been meaning to try different ratings. It is possible that I had a varying exposure compensation. I could've used both -0.3 and -0.7, either way, that should've made it darker, not brighter. I also don't think, based on where the overexposure begins and ends, that this was the issue. If I did change exp comp, it'd have been after turning the camera off then back on. I don't recall doing that at the intervals it switches to overexposure.

Cleaning how - is it something I could do at home or is it a fairly cheap and readily available repair that I could get done?