Whenever I drive on the highway after about 10 minutes the traction control light turns on. When I get off and drive on city streets the car has a heavy shift. After the engine has been off for 10-15 minutes the light will go off and the car drives normal again. When I take the car to the dealer, they say the light has to be on when they do the diagnostics, but when I drive it in, by the time they get to it the light is no longer on. Any ideas?