make sure you have the latest drivers, and the latest radiant.
might help to turn on wireframe view in the camera, and to have cubic clipping on as well. save your map, and quit/restart radiant.
happened to me all the time when i had a voodoo3, soon as i did a large operation and moved the camera it crashed so i learned to use wireframe view until i got a tnt2; then the crash happened rarely but still happened; then i got a gf2 and latest nvidia drivers and it all went away.