When I try to accelorate hard it will take off as normal up until it hits 4000-5000 RPMs. Then it sort of chokes and the rpms go crazy. It will hit about 4500 rpms and drops to 2000rpms and then back up and down. It keeps doing this rapidly, with out delay between. What could be the problem. I just had my head replaced and it didnt do this before.....But i dont think that would have anything to do with it, would it?