LIGHT!!!
If you are talking about the screw sticking out above the base of the tube, it is the focus:
http://www.videokarma.org/attachment...8&d=1257472420
Is your focus ok? you might try turning the screw a few turns either way to see if it improves.
When you play a VCR tape, you may experience "flagging"... the top few inches of the screen may appear to be waving horizontally... this is fairly common on older sets, and perhaps the Horizontal AFC circuit can be modified to correct this. A DVD should work fine, with no flagging. Play through a modulator, or perhaps through your VCR.
jr