It sounds like a vacuum leak somewhere on the engine, these old cars used to have a modulator valve that controlled shifting on the automatic trans too. there is a vacumn line that usually would come from the intake and go down to this valve on the trans, make sure that vacumn line isn't cracked or broke. If by chance it doesn't seem to shift properly, that's usually the cause! also, make sure the carburetor base gasket is tight. Spray a little either at and around the base of the carburetor, just a little bit now if it speeds up, change your base gasket. Good Luck!
Jerry Neighoff
Neighoff Services Center
Pasadena, MD 21122