Some devices have a problem that makes the gas button stop after a while. If you have this problem you can enable the Auto Throttle mode in the game's options menu. The gas will then be on all the time and you press a button to break or idle the engine. Another way to counter this problem is to let go of the gas button in the air after a jump. There's no benefit of throttling in the air in Mad Skills Motocross 2.
If you're using an iOS device that supports 3D Touch you can try turning off this feature while playing. This may improve the controls and stop them from cutting out.