Assuming all other potential issues are eliminated (and it sounds like you've done a good job of making sure of that), I would check the following:

1. Does the aperture close on its own smoothly? Almost all Nikon F-mount lenses (including this one) have a mechanical linkage to the camera body. So you can easily check the aperture's action by taking the lens off the camera body, and using your finger to manually move the aperture linkage lever. It should move counter-clockwise (when looking at the rear of the lens) (causing the iris to open all the way to ƒ/1.8), and the internal spring should return to the clockwise position (returning the iris to the stopped-down setting indicated on the aperture control ring). It should rotate very smoothly, and not require any force other than the obvious pressure against the spring.

   The aperture linkage lever is shown by the yellow arrow in the image below (ignore the red arrows; I copied this image from another answer on this site):

   

2. Closely related, is the aperture linkage lever bent, or worn? It's very short, and usually won't have much opportunity to be bent. But it's possible that there is a slight bend in the camera's aperture linkage lever that engages the one on the lens. It could be very slight, and due to mechanical tolerance mismatch, it's possible that your specific camera body and this specific lens don't physically engage together apropriately, while still not exhibiting any aperture problems with your other lenses.