after a long idle weekend...
It seems to be a video mode initialization problem, at least the first problem appeared.
Now that is set NVreg_Mobile to 2 (the only setting which changes the behaviour of X), it only draws noise (well defined noise, pixels with 'random' colors).
Today I've tried a lot of frequencies (hsync/vsync) well supported by the monitor (1280x1024 60Hz), but X does not function in a proper way.
Thanks again for answering!
huffman salutes you!