Keep the gas on. The wheels act like gyros - basic jumping.
Gas on - Brings front up Brake on - Pitch front down Turn left gas on - Turns truck to left in air brings front up Turn right gas on - Turns the truck to the right in air brings front up
I am guessing that when you hit a jump the truck gets in the air, you get nervous hit the brakes, the trucks front end points to the ground - then somersaults. Why do I know this? Because I did the same thing a million times untill I learned to either one) not hit the breaks or two) work the gas or brake to fix the in air problem.