Has someone compared the current video settings to the default settings on one that doesn't show the problem? It sounds like a Refresh Rate issue to me, but that's only a guess. Since you are getting a new one from Dell, I take it that they have already done troubleshooting with you.
Could be a Video or Monitor driver issue. Have you checked for newer drivers from Dell or from the Video Hardware or Monitor Hardware maker?
Are you using a Dell Factory software load or some other load?