Like I said, 1st, 1st, reverse and do the roll trick. This is a known issue with the T-45s, Ford has a TSB out for it. If you get your trans rebuilt by someone like t45source they will always do the reverse fix during the rebuild for you. If you can't get it to stop doing it by one of the methods I listed well... nothing you can do short of a rebuild.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.