Ghost images on screen and intermittent screen flicker.

Content shown on the screen leaves an imprint. For example, if I open a Safari window and browse for < 5 minutes and then move the window, I can see an imprint of the previous position of the window on the desktop. If I start the screen saver the imprint disappears. I also get intermittent flicker of the display. The flicker is especially prominent during startup - the progress bar and apple logo would drop down for a second and then re-appear. Any suggestions?