I have a 1990 sc with an automatic tranny it was rebuilt about a month ago and now when i get on the highway and gun it it seems like after about 70 or 80 mph it seems like it goes into neutral it just redlines and if i bring it back under 70 it will speed up again and then just do the same thing any ideas i still have warranty but i want to know or have some sort of idea to tell the guy who fixed it whats wrong so he dont try to screw me any ideas thanks