Asus m4a79xtd evo problem

Hello,
I have an asus m4a79xtd evo motherboard with an amd phenom II x4 965. I originally had an Nvidia GT8800 with a Samsung syncmaster 941. There is a video flicker square that is always in the center of my screen. It appears as though a 1" x 1" block in the center of my screen is always blanked out. I have tried all three video settings in the bios config. I have tried different drivers for the monitor at different resolutions/frequencies. I recently swapped the Nvidia 8800GT with an nvidia 9500gt. The problem has reappeared.

Both of these video cards coupled with this Samsung monitor work fine on other Asus systems in the house.
Any ideas?.....Thanks much.