Slowing down your computer sounds like a video card issue, not the monitor. Most likely driver or setup related (either the game or the ATI drivers). It sounds like what happens during an explosion or flash of light. The FPS drops way off. If those games support HDR turn it off in setup. The x800xl is iff'y for HDR.
-First strip out the drives completely with ccleaner.exe, make sure there are no lingering nvidia drivers too.
-Reload the latest radeon drivers (ATI's not any of the others). See if the problem is still there.-Set up your monitor per the link below. Playing with the settings probably has it hosed big time by now.
-Now go into the game setup and see if dynamic lighting, shadows, etc affects the problem. And try dropping to a lower detail level just for a test. There could be an incomaptibility between those particular games and your rig. Unlikely but worth checking it.
-Now armed with all this contact ATI tech support with the problem. They like to deny their stuff doesn't work unless you ask them straight out.
The X800XL drivers sucked for months but they seem to have them right finally.
Monitor Adjustment:
There are 2 settings on your monitor's internal menu to check. Video level (0.7 and 1.0 affect the brightness) and color temperature (6500K is recommended if you do video/photo work). 6500K seems yellowish at first but is actually more realistic.
Reset the monitor to default and then go here to adjust brightness, contrast and gamma properly.
http://www.nies.ch/tech/monitor.en.php
