Screen falls out on windows xp

My screen falls out on windows xp. It is a dell computer, I have called the
helpdesk and they say it is not the computer hardware since when I push on
f2, and go in the set up screen, not start up windows, the problem does not
occur. If I use another screen, it has the same effect so it is something in
the computer, system or screendevice card. ANy ideas?

Please answer in english, dutch or spanish.