Nikon D90 exposure problem?

My Nikon D90 doesn't expose correctly when using my Sigma 24-70 f2.8 EX DG. At the wide end it overexposes by a little over a stop, at about 35mm it correctly exposes, and at the long end it underexposes by about at stop. This only happens with this lens. I have tried 5 other Nikon lenses which all respond as expected.

The problem gets worse when using matrix metering. I currently use center weighted metering all the time now b/c of this lens. I know that it has nothing to do w/ my exposure settings (ISO, aperture, shutter). Instead, when using this lens, the meter seems to consistently give incorrect readings and exposures. I don't even think it has anything to do w/ aperture blades sticking as this would still at least give me correct metering. If I fill the frame w/ an off-white wall at 24mm and zoom into 70mm the meter changes over 2 stops when is should remain the same, as I'm still filing the frame w/ the same color and the same amount of light. The focal length should not affect the exposure.

I have sent the lens to Sigma and they sent it back with a note saying they made an "aperture control arm adjustment" or something like that.

Update: i just tried this lens on my D3000 (which doesn't autofocus w/ this lens) and it meters correctly. So to summarize, it doesn't meter correctly on my D90 where every other lens I have does, but it meters correctly on my D3000. Sigma seems to think the lens is fine. So is the problem my D90 or the Sigma lens, or that they are both fine but just not compatible with each other? Is there any way I can get this lens to work correctly on my D90?

Any ideas/suggestions?

Well yes, I have a suggestion. Since it only happens with this lens what does that tell you… It's the lens and has nothing to do with the camera. If the lens is pretty new contact the maker and either
get your money back or another lens.

In my experience there aren't many good 3rd party lenses out there - at least not as they compare to
OEM lenses, those made by Canon and Nikon.