Car flipping and losing wheels on start

I've been following this tutorial series about how to make an advanced FPS game but using an asset pack from a source that I purchased. I used the Unity Standard Asset pack vehicle as a guide when setting up my car, but when the player gets into the car, the car flips up into the air, the wheels vanish, and it just keeps spinning in place doing a loop-to-loop. I'm more than happy to share the scripts I have written (at his direction) and whatever else I need to share. I am at wit's end trying to resolve this, as the final game I intend to make involves the player getting in and out of the vehicle and driving around the city and countryside in my game world... So what happens: game loads, zero errors, and the player turns and rushes to the vehicle and enters it. Instantly the engine revs up, the tires vanish (they're actually dropping through the floor) and the vehicle starts flipping in a big loop, as if it's being propelled forward? The vehicle is not flipping in a turn, but as soon as the player gets in, if that makes sense.