I have a 1997 Yukon GT with a 5.7L, v8, auto transmission. When I drive for over twenty minutes and the outside temperature is at least 80-85 it slams into every gear when it shifts. As soon as I let it cool down for a few minutes the problem seems to go away. It usually occurs after I've been on the highway for a while. As soon as I get off the highway and onto the secondary roads and the truck starts shifting again it bangs into every gear. This was a problem last summer and as soon as the weather started cooling off it went away. Now that summer is here again I'm having the same problem. I recently had the transmission flushed but I haven't driven it in the heat yet.