I just bought a 1997 Z71 1/2 Ton Pickup with the 350 Vortec. The transmission shifts very nicely until after I have driven at freeway speeds for about 10 miles. Then when I come to a stop the transmission will shift very hard from 1st to 2nd, and then harder than usual through the rest of the gears. I suspected that it might be getting hot at first, but have found that if I shut the truck off for 25 seconds or so, it fixes the problem for a while. Any ideas or suggestions?