My dog use to love to take a car ride, he was very well behaved , usually slept. Now after a long road trip(last year) he gets very stressed out, jumps around. a danger to drive with. I have him harnessed in the back seat, on a short leash. It doesn't seam to matter if its a short trip or a little longer. If the car is not moving he would stay in it forever and sleep. But once the car moves, he's up and moving, he shows great signs of stress. Please help