sorry
i was working on another model so did not have the time to test it thoroughly but I did test it on version 5 ad got the same result.

I got your answer for the ghost nurbs. Select them then 'export selection. IGES format' reimport them and they should be fine. I remember having this happen to a patch model of mine once back when i was using 4.0. Nurbs seems to have some attributes that cannot be erased easily. It appears that exporting them to IGES strips them of everything but there vertice location data and curve construction data and regenerates the Nurbs surface on import. I dont know what else the surface could have that it ditches but this seems to take it back to square one and clears 90% of all anomallies.