These lenses have insides that are accessible: all you have to do is unscrew both the front and rear element sets. It MIGHT be that the escape mechanism has a bit of grime in it and cannot easily be positioned at each shutter speed change. I would flush out the entire mechanism with lighter fluid after the glass has been removed and until all the liquid does not discolor any more. It just might work and it does no harm. Then dry in a warm, dry place overnight, making sure to periodically move the shutter blades completely so no liquid remains between the blades. My oven, with the pilot light on, provides an environment that is about 110 F but you could also use a hair drier to speed things up. - David Lyga