Don’t make your wheels Balls. That means there’s only a tiny point of ground contact, so your friction has no chance of keeping you driving properly. Try Cylinders for your wheels instead.
Why do you have 6 springs? You should only need 1 vertical spring for each wheel for the suspension.
In your speed boost script how are you increasing the car’s speed? If you have too much MotorMaxTorque your wheels will spin uncontrollably.
I also start with a low MotorMaxAcceleration and increase it gradually until the car drives smoothly and realistically. If it’s too high it’s like stomping your foot on the gas every time you want to accelerate and the wheels will spin and lose traction.