Car speed physics problem, accelerates and turns out of controll

Hi for ease of explaining, I have made a video showing what I mean: https://www.youtube.com/watch?v=He1Id0Q3H7M

Basically, once I start playing the game, everything is fine, but after a few minutes the physics seems to get out of whack. What that means exactly I can't say except that when the car accelerates unusually fast, sometimes when I apply brake torque the car still accelerates! Sometimes it turns for no reason as well... the ground plane is completely flat, I have readouts that you can watch in the video, I have max torque and max steering angle you can watch as I play the scene and ultimately see an example of what I mean in the video linked above.

I have here a GIST of the CarControl.cs doing all the calculations for the car. https://gist.github.com/victorbstan/e5903829576eaf6ce5e3