Take it to a new dealership and GO WITH THE TECH when he/she takes it for a drive and specifically let them know how to replicate the issue. You are under warranty and should not have to pay for them to fix this issue.
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 ****.