Occasional artifacts on screen

Time after time artifacts show up on the screen. Sometimes they appear for a brief moment, sometimes they just stay there until the entire view is refreshed. Artifacts do not appear on screenshots. In general, they are not disturbing, but perhaps they indicate some more serious underlying problems.

The question is whether this is normal behaviour? I've still got Applecare until March, so I want to get it sorted before the end of it.

System: MBP 13 mid 2010 (2.4 C2D, 4gb RAM, 250gb HDD, 320m, updated Lion)



P.S. I have reinstalled Mac OS, but the problem persists. Also, the computer seems to have a track record of issues- it has been in warranty repair 5 times already.