I did have multiple monitor entries indeed. I got rid of both and rebooted and it selected itself... there's only one now. However that didn't fix the problem.
My videocard is a Voodoo5 5500 AGP, but I have the feeling this is a monitor problem.
The patch seem to get better or worse, but is still there. it doesn't stop me from seeing what's on the screen, but spending nearly £300 on it I expected it to last a bit (read that "a lot") more than two years...
Cricket, you were saying this could be a cable problem... Please explain... at least I'll have an explanation if nothing else - the cable is built-in so it cannot be changed...
Thanks Cricket for your answers - I am still open to suggestions.
Cheers