Transmission Shifting Hard
(First Owner) I have GMC Sierra 2006 2500hd 6.0L
4 speed automatic transmission with mileage
290000km. When I start the engine everything work
fine until I cross 120 km/h the problem start here,
the transmission start shifting hard from gear 1 to 4.
And this problem disappears when I restart the car.
When I drive slow this issue not happening!