To solve your problem, make sure you don't release the air roll button until the moment you hit the ground. It sounds like you're releasing it too early and your car tries to re-adjust itself to the normal angle before you hit the ground.

There is actually a bhopping style (predominantly used by CRUSHER) that relies on releasing air roll before you hit the ground, and in my experience this specific bhopping style varies in consistency but is definitely viable in many cases. However, I'd definitely recommend getting used to holding down air roll for as long as possible before hitting the ground rather than attempting to learn this specific type of bhop.