You would have to detail the video card settings and try the HDMI connection as well. I can see this failing if the color depth was set too low on the external connection/monitor. This area is not consumer friendly.
I recently bought a Samsung S27A550H monitor and attached it as a dual monitor to my laptop. I'm using Intel(HD) graphics, running display fusion to extend monitors. I've been getting very weird behavior on the second monitor.
Chrome automatically opens some pages in a second/background tab. For instance, the confirmation page when posting on Craigslist, or displaying full-view pictures on Facebook.
If I switch Chrome to the other (laptop) monitor, it all works fine.
Also, the sliders in Adobe Fireworks/Photoshop color picker or font-size picker don't seem to work on the Samsung. If I switch to the laptop, everything works fine.
I am connected in Analog (VGA) mode if that makes any difference.