1999mustang2000

Ok so some of y'all have probly seen my other threads from where i put in a high stall torque converter and a performance valvebody. Ok, well i got all the bugs sorted out except one and thats when im above 35mph and let off the gas the car slows down and kinda jerks likes it trying to down shift or like the brakes are ingaging and disengaging idk? What could be the cause of this? i have the 4R70W transmission.