Have you had your thrust on the rear axle checked. That will make your rear end go either left or right consistently. That and the fact that you are not modulating your throttle. I had the same issue.
'16 Eco-Boost Premium, White, Performance Pkg, 6R80 (again), Pwr Ebony Lthr with Nether Region Cooling, Adaptive Cruise Control, Enhanced Security Pkg., Reverse Park Assist., Big Worm OTT Graphix.
2014 GT Premium 200A, Black on black , leather , 6R80, Nav, Security. Pro Cal, HT catback exhaust 12.54@112