HELP !! must RE-RENDER already RENDERED clips...

Dear friends,
i'm working on a project based on a PRO RES422 timeline and it's happening something very strange....
I have 40 already rendered clips in my timeline when I need to have a break I launch rendering process pressing ALT-R on my keyboard, my Mac starts to render and iI go to drink a coffee...when I come back I find everything not rendered also the clips that have been rendered yesterday.....Sometimes appears:" SMOOTHCAM FAILED TO RENDER "

Are there solutions?
I tried to:
-erase all render files and re-render again
-make a new project
-revert the project AND SOMETIMES IT WORKS...it means that everything is correctly rendered !!!!

Before deleting preferences, any suggestions...????

thankyou

Alvo

MAC INTEL dual quadcore 3ghz
8gb ram
macos 10.5.6
fcp 6.0.5