x20 problem: black border after using external monitor

[dogpile]

hi folks,

i used to plug in a external monitor into my x20 and worked with it.
after plugging-off, i switched into normal mode (Fn-key + F7) and everything was ok. but then i tried to watch a video at 800x600 resolution, and there is a black border around the screen, means it does`nt use the whole 12 inch of the tft...
can anybody give a workaround or something?

thxn

christian