some times i have the message "composition was too slow and was going to be disabled" and the 3D effects are disabled. After logout, the 3D comes back, stilll the next time.

i have nvidia 7600
x11-driver-video-nvidia-current-180.51-1mdv2009.1

I'm not sure but I think there is a option somewhere to disable this.

I press Alt+Shift+F12 to enable/disable compositing effects. If you don't have changed this, this shortcut should work.

theshortcut did nt works because i use alt+shift to change the keyboard layout!
i changed the shortcut and now i can enable/disable the 3d from the keybord shortcuts

Solved, no - there needs to be a way to turn off this auto-disable feature - it does my head in to no end.

System settings -> Desktop -> Advanced -> Disable functionality checks.