My GT with 224k shifts hard from 1st to 2nd only. I tried it in sport mode and manual mode and still does the same. There are no fault codes. I used my Cortex to switch between stock and Performance tune and it didn't make a difference.The tranny fluid and filter were changed a few months ago. This car was never abused or driven hard. I explained my mechanic who is familiar with the 6L80 and he said it might be a cracked drum.