Because the mod was programmed to have a personality. You may have driven it too hard or damaged it too much.
Then I guess just drive smoothly until it will let you go faster. Or, you know, you can just do as someone else said and take the easy way out.
This car has it's own personality. i doubt any sane personality would like it if you kicked it This game isn't just about crashing cars; it's a soft-body physics simulator that's trying to be as real as possible. The crashes are a side-effect.
This particular mod is based off of a car in a movie (simply search for "Herbie" and you'll find it) iirc, which is why it even has a personality.
Did you watch too much Camodo Gaming on YouTube? This is an in-depth vehicle simulator, not just a car crash game. If you don't like it then don't use this mod.