Hi Davey - if saving as ‘Geometry only’ still shows the problem in the resulting file, it may be that Rhino is trying to mesh a bad object. Try opening Rhino in Safe Mode and then open the file from that instance of Rhino - does it still choke? If not, ClearAllMeshes, make sure all viewports are in Wireframe and then open the file in a normal Rhino - does that work better? At any rate I suspect, for now at least, a bad object or at least an ‘unmeshable’ one. Try SelBadObjects and see if anything lights up. If so, Export and post here or send to tech@mcneel.com.
-Pascal