thats funny, my own CRT TV set does this. Everytime it is on a still picture, the picture on the screen vibrates a little, but it gets smooth once something on the screen moves. ie when I play Super Mario World on the TV set while Mario moves, the picture is fine, and is fine when Mario stops moving, because of the time counter and stuff is still animating, but when i press pause, the picture on the TV screen vibrates slightly.
It also vibrates slightly on the AV, like the menu screen of my DVD player, it vibrates slightly on that but the picture is perfect when watching a DVD. (panasonic DVD player and a Grundig TV)
I think maybe it depends on the make or model of the TV set, I am not sure. When i told my friend's dad who is a TV technician he said it can vary across makes and models.