Why would the temperature gauge on a 1989 Toyota Corolla manual rise when driving faster and go back down when you slow down?

Answer

Have you checked the fluids (e.g. oil, coolant, Transmission Fluid)? When the car is working harder--as in driving faster-- it generates more heat. The car sheds heat through its fluids. If the fluids are full then it sounds like the coolant is restructed. Take it to a radiator shop.