cpu overload on mac pro 2.66 with nuendo 5.5.3

i have a cpu overloading issue very often in Nuendo 5.5.3 running on a mac pro 2.1.1 2.66 dual xeon with 12 Gb ram, in both Snow leopard osx 10.6 and lion OSX 10.7. Sound card MOTU PCI 424 with 2x 2408mk3

The activity monitor seem to be almost near 100% and a pop up message window with CPU overload appears.

I have already tested the mac pro with other DAW applications and everything went fine on this issue.

Buffer setting?