And one more question: your sugestion to reduce scene size is pretty intersting but I don't know what should I reduce exactly, because in my scene there is a lot of nodes?
Z-buffer problems occur when geometry objects situated very close and very far from the viewer are simultaneously rendered in Cortona.
So:
1 don't place any geometry close to camera (HUD)
2 don't place any geometry very far from camera (scene size)
3 use NavigationInfo parameters to improve Z-buffer accuracy (http://www.parallelgraphics.com/developer/products/cortona/extensions/navinfo)
Problem by using ProximitySensor