Internal v. external is not the same as onboard v. discrete.
Starker, do you have a discrete video card added to the mobo or are you using integrated graphics on the mobo?
If not sure, go to Start/Programs/Accessories/System Tools/Sytem Information
Select (and expand if necessary by clicking the plus sign ahead of the name) Components/Multimedia/Display
Then copy and paste the top 7 lines and it will tell what you have.
Did this problem just start on its own, or result from any special actions - including any hardware or software changes or even after moving the computer or monitor or reconnecting any cables?
Have you tried using a different monitor and/or cable to isolate the issue?
Does this occur both when starting the system and during operation?