@dnelsonfl:
Many thanks for your help!!!
I've discovered the problem... I needed to close a jumper near the "Sync" chip on the board.
What can I say...? Amiga with a graphic card is simply fantastic! :-D
With my monitor, I can use all screenmodes up to 1024x768 8bit. Unfortunately at 1024x768 16bit doesn't work so I've chosen 800x600 24bit... very very nice!!!
Thanks again!!!
Alex