I have a vague feeling that I had the same problem but that was a few years ago. I also have a vague certainty that the cause was determined. I'll try to remember what if if there was a fix. To get another obvious one out of the way in the mean time though, has it been doing it the whole time you've used 2004 or did it start recently? Did you install anything around the time it started? You also might check your video card driver to see if it updated recently and try rolling it back. You might also want to try reducing the color depth or resolution(nagging feeling that this was my problem). A lot of people, myself included, have a tendency to set the resolution as high as they can while still getting a picture. A lot of times, a resolution that the video card and/or monitor really can't handle well, can be selected and strange stuff happens.
