dual screen destroyd xorg.conf

Notebook Nvidia 8600 graphic, screen resolution 1680x1050, external monitor 1280x1024

1. installation kubuntu 8.04 beta works fine, propietary nvidia driver is ok.
2. switching on external monitor, mode copy 1st monitor, is ok.
3. screnn on the external monitor is too big, screen is scrolling even if the internal monitor is switched off
4. trying to set the external monitor to 1280x1024 dpi, screen is still scrolling. (Only the external monitor is activated)
5. switching on both screens shows a resolution of 1680x1050 with the external monitor scrolling sideways and cutting top and bottom
6. reboot, screen is set to 640x480, nv mode. No native mode
7. system settings - screen says module is broken or not installed. Running dpkg-reconfigure -phigh xserver-xorg brings back the screen settings.
8. switching back to native nvidia driver is not possible, resolution under nv driver is limited to 640x480
9. etc/X11 - copy Xorg.conf.4 to Xorg.conf brings back the native nvidia driver under 1680x1050

I cant use the external monitor unless the screen resolution is the same as on the internal display.
Running Windows XP the screen works as it should be: running the internal screen shows the notebook display running on 1680x1050. running the external display only shows the external monitor running on 1280x1024. running both displays shows the external display on 1280x1024 and the internal display on 1280x1024 with black bars on each side.
Is there a chance to get the external monitor working under 1280x1024 without destroying the xorg.conf? I really need this.