Lanscape spline - 'bouncing' effect where segments meet.

I have a mesh attached to a landscape spline which has an associated simple box collision.

When an object moves over one spline segment to another (they are being used to model a road) at high speed it ‘bounces’ up. It’s as if, where the two spline segments meet the collision boxes don’t line up exactly creating a small lip. This lip acts as a barrier causing the object to hit the segment and bounce up.

How can I stop this effect? I would like my spline to be perfectly smooth. I’ve searched high and low and can’t find a way to get this to work. Thanks.

For anyone interested I think this problem is an instance of the issue discussed here - Rolling ball jumps on flat surface - Unreal Engine Forums