Sienna transmission not pulling when warm?

We had to put a new engine in the vehicle and once the engine was put in and the mechanic drove it down the road it would drive just fine while cold, but would shift really hard into 3rd gear and then once the engine and transmission was warm it completely stopped pulling the vehicle. The mechanical has now put in 4 transmission and a new wiring harness and continues to have the same problem on all 4 transmissions. We are not completely at a loss for what could be causing this. We have talked to a couple other transmission shops and they too have not been able to diagnose the situation. The only recommendation that has been thrown out is that there might be a sensor that once the vehicle is warm is throwing a signal to the transmission telling it to stop running but no one seems to know which sensor it might be and the vehicle is not showing a check engine light or a code of any sort. Any suggestions as to what might be causing this would be extremely helpful. Thank you for your time. -Paul McPherson