Marcioguitar is absolutely correct - having ANY incompatible plug-ins installed, even if they're not used in the session, can cause all kinds of problems.
Kontakt Player is one of them - it's not been updated for Leopard yet.
Native Instruments compatibility with Leopard
Verify the versions of plug-ins you have installed against the
Pro Tools Leopard Plug-in Updates page. Remove any incompatible or suspect plug-ins, then trash prefs and test again.
You should also see our
VI performance info sticky for more ways to improve performance of VI's in Pro Tools.