I bought a 2003 Yukon with a 4.8 engine and a 4L60E transmission. I live in the Adirondack mountains in upstate New York and I have a problem that I have never heard of. Got the cruise on 60 and I'm heading up the side of a mountain on a road and my truck will downshift from overdrive to third and then down to second in a matter of 5 seconds. I've never heard of this in my life, would anybody have any ideas?