Have you checked that the distance scale revolving knob in front of the distance scale window is set on 80mm before you fit the standard 80mm when changing from the 180mm lens? I have just tried it with mine after reading your posting, and this may be the cause of your problem, if not , I would remove the folding focusing hood, and check that the screen is seated correctly this is a possible cause of focus error , failing this as Dr. Bob says it may have been inexpertly taken apart and reassembled by a previous owner, or suffered impact damage resulting in it being put out of collimation, which would need reassembling on an optical bench by a repairer, it would probably be cheaper to get another one . If you could find a shop or someone who has one and try theirs