Just went onto the DELL site and found that it's a Flatpanel Monitor (LCD). These monitors usually have a maximum vertical refresh of around 75-85hz.
It may be possible that Linux is exceeding this rate.
I'd check the monitor manual or CD-Manual for the refresh rates for your monitor and add/modify your XF86Config file.
LCD monitors are VERY touchy when it comes to refresh rates.
This could be your problem.
Hope this helps.