Walter,
Clipping - its a real pain in the neck sometimes !!!
A few things to try / check on your model:
Are you working a long way away from the origin (0,0,0) ???
SU doesn't like this and it's one cause of clipping.
If this is the case in your model, turn all layers on, unhide everything, select all, group all, move all your geometry back towards the origin.
Does your model cover a large physical area ???
SU is not particularly keen on this either - again a cause of clipping.
Turn all layers on, unhide everything, check to see if you have any stray linework etc miles away from where you are working.
............
Hope this helps
Regards
Howard L'