What is probably happening

What to do

Video card is underpowered

Try cutting back on the number of colors displayed. Chances are, you won't miss millions of colors vs. 16 colors. If this fails, replace your card with 128-bit card or equivalent.

If the problem rarely occurs, you can often use the page up or page down keys to force a refresh. If that doesn't work, just reboot for a given instance of the problem.