Firstly, sometimes when you're up against the barrier, you can be dragged HARD to the left or right by an unseen force that can result in you going off the side of the platform with nothing you can do to stop it. Secondly, when you hit the barrier at a particularly high speed, you have a chance to hit it and have the impact of it launch you JUST high enough to sail right over it. No ramps or dips in the road needed for this, sometimes you hitting the barrier causes you to get carried over it. Thirdly, sometimes when you're up against the barrier (in a stopped state), you can sometimes be carried over it by literally nothing. No psions in sight, jump button being actively avoided, and sometimes the game will just decide to screw you over by sending you over the barrier anyway. Solution? Raise the barriers a tad, and fix the "drifting rapidly to one side" thing, because I'm pretty sure that's not an "intended mechanic".