BenGoble, 25,000 faces shouldn't be a problem. What version of MS are you using? (V8iSS2)
Where did the mesh come from Civil3D, 3DS, or Rhino? I have had problems rendering models and meshes from other programs.
Can you recreate the surface using InRoads, GEOPAK, or Data Acquisition?
Create a new 3D file from seed.
Reference your bad file.
Make sure the reference file falls inside or near the Solids Working Area.
Use the Merge into Master command.
That should leave behind any fatal or corrupt elements in the design file.