I'm not sure if screen tearing is the correct term for whats happening, but I don't what else to call it.
It like starts juddering and the text on the screen goes a bit blurry; the text also looks like it out of place.
(Sorry if I haven't been very descriptive, but its hard for me to explain)
It's an ASUS 24" inch monitor (VGA), and I've had it about a year 4 months.
Could be the refresh rate of that video output is not matching the rate of the monitor. If this is a LCD monitor (which probably is, considering the size), it has a native refresh rate, usually 60Hz. Make sure the video card outputs the same rate. If you have the option to connect it via DVI instead of VGA, that is much better for this type of monitor.