I have a 2005 4.4 w/ 62 k miles and have the same problem. I live in Colorado and it only happens when I head up to the mtns, so I'm assumiing when there is more of a load on the vehicle. Same issue with the dealer, they can't produce an error code, so nothing can be done. I emailed your experience and they asked I bring the car in again. Of course they could not reproduce since it's relatively flat where they drove the car. I've told them numerous times it only happens when headed up 1-70 toward mtns and of course their response was we can't drive that far. It's really only about 20 minutes to get there too! They noted the issue in their database and said that they will replace the mechatronics unit if I can reproduce it with a tech and I have till 75kmiles. I really do appreciate them doing that even after the warratnty has expired. Now I need to try to find a road / highway close by with a steady incline that I can get up to highway speed. They tech can only be gone for no more that 20 minutes though.