Just a guess, but check to see if your secondary display is set for NTSC or PAL (possibly a third SECAM), depending on where you are located, it may need to be set to a different setting. (In the US, NTSC is the necessary setting) You can check the video broadcast standard for your country
here.
This setting should be located in the display properties section of your control panel. There should be a device selection or something like that, where you can change this setting. If I were you I'd check the manual or call dell support, they could probably direct you better on where to find this setting.
Hope it helps,
Flip