Recently my camera will take pictures fine, but sometimes between pictures the aperture setting changes from showing (say) "9" (to represent f/9) to showing "--". After this, if I take a picture it turns out completely black.

Even if I turn the camera off and on again, most of the time it still has the issue; the only thing I have found which works is to remove the battery, then put it back in: this clears the issue. Any thoughts on what could be the issue?