Sounds like the TV or (throttle valve) is sticking. There used to be a cable that went from the throttle body or carb to the transmission. This cable controls a valve that works against the govenor pressure to determine when the trans shifts. TV gets it's pressure from the line pressure of the trans, the more throttle the more the TV opens to prevent the govenor pressure from shifting the valve. This cable can be adjusted to control the shift points. The govenor could also be sticking or be bad.
