Yeah, I had one or two problems with this mission too, it's not easy with the car they give you. If I remember, they tell you to knock him off the road, but what they really mean, is kill him. So don't be afraid to drive-by his ass. Drive up close on a straight and let rip with a gun.
Other than that, I'm not sure. It really is quite hard and having to repeat that first part is annoying after a while (However cool it is). Good luck.