It really helps people answering your question if they know exactly what your OS, ArchiCAD and system specs area - good place for this is in your signature.
You say you are on a new Mac Pro .. running Leopard? This sounds like a problem many people (including myself) are having under Leopard. Remember GS has not released an officially Leopard-compatible update yet
cheers,
Owen Sharp
Design Technology Manager
fjmt | francis-jones morehen thorp
iMac 27" i7 2.93Ghz | 32GB RAM | OS 10.10 | Since AC5