Are you sure the flywheel teeth are not worn badly and causing a drag? Upon inspection you will notice, as I have, that the flywheel will have three areas of noticeable wear where the starter drive engages. That is about all the wear areas that could change over time...of course if the bolts from the bellhousing to the block have loosened that is a different matter. I have seen a lot of weird issues on the cars that rolled through the wrecking yard I worked in and I have not seen everything yet.