Ah, sounds the shutter blades. The one I am working on has a sticky aperture and shutter blades. I noticed that sometimes the shutter blades get stuck ... if I put slight pressure on them, it trips ... so this tells me that there is substance on them to make them stick. The easiest way of getting to the shutter blades is by removing the rear lens group ... then wash'n work the blades. It is not easy to take this apart to do a real cleaning.