Given the information you have supplied I would agree with Brian (blw) that it does not sound like bracketing. Bracketing just happens to be a common cause of exposure problems like yours and as you found out, it doesn't work like many people assume. But again, that does not sound like your problem.
Given your EXIF data (exposure)shows no change, it would seems to be a mechanical aperture problem. I would certainly try the tests Brian (blw) mentioned. In addition, I might try the following:
Aperture preview button Set the camera to something like f8 in fairly bright conditions. Zoom to 24mm and press the aperture preview button and look through the viewfinder. Do the same with the zoom at 70. Does it look different. Do it with a flashlight looking into the lens. Do you see the aperture stepping down?
Wide open Set the aperture to 2.8. Shoot at 24 and 70. It shouldn't be brighter at 70 since it can't get any wider.