Really Angering Me! (Screen Flickering Off)

I have two computers set up so that they both share a monitor, and recently, my monitor has been flickering off every so-often, so that I am forced to either turn the monitor off and on, or switch from one computer to the other...

It's really annoying! I've had problems with this monitor before (My monitor will randomly tweak out, and I have to force-shutdown the computer)

Please help! I know it isn't video card drivers, because it is happening to both of my computers, (HP Media Center 1.0GB SDRAM, 630 Intel Pentium 4 Processor, 250GB 7200RPM Serial ATA Hard Drive/Compaq Presario 512GB SDRAM, 2.49 GHz, INTEl CELERON CPU)

 

Please help!