Some connection issue. A technician would, well this is not a TV repair forum so I'll stop here and write you need to arrange for service. That tapping could be classified as abuse which you don't want to continue doing.
I have a LN52A650A1FXZA flat screen. It has started to develop vertical bars and partial blurring of the screen. Sometimes it corrects itself and other times I have to tap the screen to make it work correctly. After tv is running and warmed up it doesn't happen the rest of the time it is on. Anyone have any ideas as to what is wrong?