[boarder_punk]

In the 4T60-E there is a PWM solenoid that controls how hard the shifts are, it does this by sort of "cushioning" the speed at which the fluid applies the bands. It is possible that the heat is affecting this solenoid and causing it to malfunction. The torque converter may have a part in this, it may be causing the tranny to reach higher temps or just get hotter faster; but that is unlikely. In my opinion it is not likely to be a valve body problem, especially if you found no metal shavings in the pan. If a valve were sticking, such as the throttle valve, it would most likely not shift into a certain gear at all.

So you are thinking about wyotech, I recomend it. I finished the auto core classes and am now in chassis fab, soon to be in street rod. In drivetrain, you actually tear down and rebuild a 4T60-E as a competency, among other things of course. I learned a lot of good stuff out here.