Try cleaning the aperture again.
I say before you buy another, take an hour or two and clean the aperture again, like REALLY clean it. Use an ultrasonic cleaner or some mild detergent. Use Pete Ganzel's directions and clean it real well, because even a little oil can cause big problems. A couple of my 50's use to get stuck once wide open at 1.7, but operated freely from 2 to 22.
Try this: take the mounting ring off and operate the aperture spring mechanism independent of the aperture rod, and if it swings back and forth with snap, then the spring is probably fine and the blades just need some cleaning.
Let us know.