From your email this does sounds like something random and sporadic that does not have a pattern. Has this started recently, or is this on a new machine. Has anything changed such as a font you did not use before, or an image or strange file format some of these file have. A corrupt/damaged image of something like a cmyk jpeg(the format was meant to be RGB and some programs occasionally act funny) can cause something like this.
Are you able to solve this by possibly toggling preview mode Command Y(Ctrl Y Win), view >> overprint preview. The preview almost looks like a cable is loose from your monitor to computer. Clean your computer with a can of compressed air and focus on any dust laying on the monitor card and cable connections outside the computer.
Sorry for all the wild guesses, but if you can tell us more about your system and any patterns you find, that can help.