From Samsung Plasma TV FAQ
Flicker, or dynamically changing brightness or gamma, occurs in the mid-grey to black areas of the screen. This problem is most noticeable in an interview type situation and can be observed in late night tv talk shows. If you have the problem you will notice the background behind a interview subject dynamically changing brightness or gamma giving the appearence of flicker.
This problem is observed in the mid-grey area and was first reported by serveral users in November/December of 2007. This problem is related to a bad logic board. Place service call with samsung for the fix. Logic board SW 71115 fixes the problem.