You state your lens setup; what camera body are you using?
I would investigate the possibility some light kicking back through the viewing system. With the camera empty, take off the lens and with the shutter on "B", lock the shutter open. From inside a darkened room, point it outside to a bright scene and look into the empty lens mount. Check for light bleeding in around the reflex mirror; maybe the foam seals around the reflex mirror have degraded and are allowing light to enter the exposure chamber from the eyepiece.