This sounds like video card issue. Have you tried running a bios recovery?
BIOS file name DP0572P.BIO
Check with another video card or update your graphics card's firmware.
infact I tried replacing the graphic card with another and that one worked super fine..
i've almost changed every other hardware component and finally i found that my monitor Samsung 943 NWX 19" LCD was the real culprit..!!
it's mainboard had some problem and that's the reason why it didn't even support it's native resolution of 1440x900..!!!
pls excuse me for the late reply..!!!