Tried with an other lens ?
Most likely the linear motor that drives the aperture needs a clean-up at its most optimistic. If you're adventurous just sit down , study how the lens is assembled and with common sense you may be able to open , clean and assemble. Lenses are pretty straightforward constructions. But is that to scary , which is understandable contact the German repairer : paepke-fototechnik.de/ they usually respond in time with advice.