Photographers and camera enthusiasts

I'm a photographer and I'm at a loss for what is wrong with my camera. I use a Nikon D80 and my primary lenses at the moment are a Nikon 18-135, Quantaray 55-200, and a Nikon 30mm. ( the newest and my favorite.) For a good while now, I've been having this problem with only the Nikon lenses. It started with the 18-135 where I'd put the lens on and for one capture, it would be fine, but my next photos would become darker and darker the more I took. When this first happened I took it for repair and the first time they said nothing was wrong. The second time I took it in they replaced some blades on my camera that did nothing. It now does this with the new Nikon lens, but never with the Quantaray.

Seems to be a connection issue with my camera, but it's so frustrating, I have to keep popping the lens off to get good photos. I can see my aperture getting smaller (as I've examined by taking photos at myself) and it won't open back up.

Anyone know the problem/what I can do to fix it?

Since it's happening on two lenses, I suspect the body. How many shots would the darkening take place? 2? 3? If more than three, take your camera and lenses to an authorized Nikon service center. If you already went to one, look for another.

If it's 3 or 2 only then the 4th would come up fine then you may have left bracketing on. Check your camera's manual on how to disable it.

The Quantaray is not affected? Does it mean you'll have to switch to that brand? Pray you won't have to!