Ok my 1969 chevelle is WAAAAAAAAAY fast. However, lately the acceleration is acting funny. While in third gear, (3 speed manual) it accelerates pretty quick when i really step on it, however after a few short seconds it actually starts SLOWING DOWN!!! It feels almost as though i am down-shifting, its the same type of feeling. It is really weird.. And then all of the sudden it will start accelerating again. I dont get it!! HELP ME!!!!!!!!!!!!! Thanks!