NIkon 17-55 2.8 sudden darkening

Hello!

I have a problem with my lens. When I'm shooting with at more than f2.8, sometimes there's a sudden darkening (that I can observe through the viewfinder) from one photo to another.

Let me be clear: these shots continuous (less than a second between them); there are no changes in light; it's the same subject or landscape; and the f number and the shutter speed is the same.

First, I thought that it was a problem with the camera (Nikon D300), but I've tried other lenses and they work ok. I still have to try with my other camera (the Nikon D800), of course, but I wonder if anybody has experienced the same problem! I will try to show two photos of this problem here!

The "normal" one! ISO 200 f10 1/400 17 mm - 10h 47m 43s January 17 2013

The "dark" one! ISO 200 f10 1/400 17 mm - 10h 47m 43s January 17 2013

Thank you!