From the sounds of it my personal guess would be something transmission related. It is possible it's not downshifting properly, or a problem with the torque converter perhaps being warped and catching, although that seems unlikely. If it is the tranny it could be the computer or the tranny itself. It does sound to me though like what happens when you leave a manual in too high of a gear while stopping, which is why I'm guessing the automatic might be doing a similar thing.