Thank you for your reply Mr.Biscardi.
It also happens with out point in the middle of the clip, with regular TC.
I tried it on 3 MacPro recently upgraded to Snow Leopard, with Kona 3 (driver version 8.0) and FC 6.0.6
I also tried to throw FC and QT pref files.
The problem remains.
Thank you for your help.
Giac