I have been experimenting with curved tracks, generated in Blender and exported to fbx files, and I see something very similar. My solution was simply to give the mesh a higher poly count.
I don’t think there’s anything funny with collision going on, because my meshes have no extra simplified collision mesh or anything like that. I’m pretty sure the engine is just using the raw mesh data for colliding the car along the track.
How many triangles are in your meshes? Are you sure it’s high enough? Have you tried simply adding more sub-divisions of the curve in maya, and thus ending up with more tris in the final asset? I would give that a shot and see what your results are. (From the picture you posted, it seems that there aren’t a whole lot of triangles in your mesh.)
Also, having some kind of smoothing on the camera can make a difference. When the camera shakes, it’s of course really noticeable. But when just the car shakes (and the camera stays smooth), then that is much more acceptable visually. Experimenting with the smoothing on the SpringArm (the component I’m using in my car currently) or some other form of smoothing could be worthwhile, if you’re not doing that already.
Can you post a video somewhere of the “lumpiness”? I could have a look and see if it’s similar to what I see.
Please keep in mind that I’m new to Unreal, so I may be doing it wrong. I’d love to know if there’s a better way.