Ron, you wrote your model is a very big one. I had the same problem with a very big model what used very big polygones. I think it could have something to do with the Z-buffer-problem: If a vertex is to far away, the depending polygons are not drawn exactly or dissapear sometimes. Try to scale it down and have a look then. If the problem dissapears, scale down the whole level.

cu Lutz, who hopes it works (let us know!).