If your monitor goes blank, does the power LED glow steadily or does it blink? If it glows steadily, then check your monitor cable.
If it blinks, check to see what power-save options you've set for the monitor under Control Panel>Power Options. Select "Never" under Turn off monitor and make sure your monitor refresh rates aren't set too high (can be checked by right clicking the desktop selecting Properties>Setttings>Advanced>Monitor>Refresh Rates
If this is right, check to see if the fan on your video card and on your CPU are spinning properly. Check your system temperatures by downloading Motherboard Monitor here:
*mbm.livewiredev.com/download.html
This problem can also occur if there is a sudden change in the power supply to the computer. Check if your UPS is working fine and there were no sudden sags or spikes in the supply. (You can notice this if the lights go dim or suddenly bright with a clicking sound)