Another thing to check is to make sure that the AI tab is following the aperture ring freely. If it's sticking in the all-the-way-photographers-left position, I think that would usually cause underexposure not overexposure, but other odd things could happen. If the shutter speed is changing smoothly as you change the aperture, it's probably ok, but you can watch the AI tab move to be sure. Also, just to be obvious, make sure you don't have any exposure compensation set on the ISO dial.