Alright, I suppose that's out of the range of possibilities then.
The only other things that currently come to my mind are materials and origins. If you have any material effects (i.e. FFP effects or shaders) applied to your entities, I'd try to disable them in order to track down the problem. As for the origins: You might want to check the origins of your entities - if they are in a position to close to the camera, I think it likely that the whole entity might be clipped. I suppose I don't have to go into detail since you seem to know 3dgs's basics
But if I were you, I'd make sure all the origins are in the right place.

Currently, that's all I can come up with... keep us posted