Optiplex GX520 Display Problem

I have an Optiplex GX520

Windows XP Pro

1Gb RAM

NVIDIA GeForce FX 5500 Display card running dual monitor setup

I have recently begun getting what i would describe as video curruption.  Program title bars disappear and programs do not draw themselves correctly.  Internet explorer, outlook, messenger, and all other programs are affected by this.  I have reformatted and reinstalled the operating system only to discover the problem coninues less than a day afterwords.  The computer cannot be left on more than 8-12 hours without rebooting to correct the problem.

Any help i could recieve on this issue would be greatly appreciated.