M992 text clarity problem

I have a 4 yr old Dimension 8300 with a M992 monitor. Over the last several months I have noticed that, once the monitor has been operating for about 15-20 minutes, the text becomes incresingly harder to read ( almost "fuzzy"). If I turn the monitor off after use, the text is clear and sharp the next time I start up; but then the "fuzziness" returns about 15-20 minutes later. I have noticed that, associated with the degradation in text quality is a kind of flickering, "herringbone" pattern that I can see in those areas of the screen which are "white". Is my monitor going bad; or is there just some adjustment that I need to make? If the latter, what kind of ajustment, and how do I do it?? Any help would be appreciated.