Make sure the resolution and refresh rate are correct. Whatever your monitor res is 1920x1080 (maybe?) and 60Hz. I've used my HDMI cable to my TV on my 5870 and for some reason it wanted to use 30HZ instead of 60 and was all choppy.
The resolution is set at 1440x1050 which is not my monitors natural resolution, could this cause the problem that I'm having?