Mine does this and not sure why, I did figure that when mine does it, it will have a pending code. Not sure what the code is bc I never have my scanner. It doesn't trip the cel and won't show up in the aeroforce but as soon as I hit clear codes it goes into lockup and shifts normal. If I'm driving in town I always leave it in 3rd bc if I have it in drive it's always down shifting. Not sure if you're having the same issue but you might check pending codes.