Sometimes the graphics on screen are not crystal clear. It is an intermittent issue. Anyone know what causes this or what news to be done? iMac 27"

Have an iMac 27">OSX 10.7.3>3.1 Ghz Intel Core i5>16 gig ram. At times the lines and font are not crystal clear as usual. A line for instance looks like the ink is bleeding and therefore not clean. Also get a whit line in desktop picture. Switching windows, screens will clear the issue. Anyone know what is the issue and how to fix?