My bet is you've got ground enabled at 0 height in the sky/realistic sky, and the plane is a 0 height.
It creates a conflict, thus sometimes the ground is shown and sometimes the plane. EDIT: Yeah I was just going to say that it seems that it's a terrain which goes under the sky's ground or an anything grooves plane (it has displacement anyway, looking at the intersection with the buildings' sides).

Message edited on: 04/08/2005 08:32