I find that in the race missions, you don\'t always need a superfast car. Try to avoid full throttle except on pavement and if you can get into 1st place, you can usually stay in 1st by just taking it real easy and focusing on staying on the road.
It takes me about 4-5 tries for each race. That first mission for Zero on the roof was a bitch though. At least a dozen tries until I got it. Stupid manual aiming joystick thingies!