Falling through ground when colliding with hovervehicle in T3D 1.1b

Hey, so in my latest problem with my ongoing project, I have a hovervehicle working perfectly on a perfectly flat ground plane. I have it working as a top-down space sim, and have figured out the camera stuff with much help from the community here...

Now I've added some "planets" to the mission, to have something else to fly around. But now when I collide with a planet, the hovervehicle falls through the ground most of the time (sometimes I can collide 2-3 times before it falls through the ground).

I have tried tweaking various settings in the hovervehicle datablock, trying to adjust it, but nothing seems to affect this falling through the ground behavior.

I have yet to modify the vehicle itself, I have modeled a ship, but haven't figured out how to setup all the additional layers for the torque exporter for it to work properly as a hovervehicle. So this one is one I found in a resource somewhere around here on the forums. I do admit it could simply be the collision geometry of the vehicle or something, but since I don't have another known working hovervehicle and am still somewhat unsure that my own efforts will result in a known-good vehicle, I want to ensure it's not something in my code making it unstable first... (so that then I can worry about building out my own hovervehicle and know that if it's screwy it's the model not the level/code).

I hope that makes sense.

Anyway, any help or suggestions would be appreciated.

Thanks!