Are the lines only at the top of the screen? Is there any "fold-over" of the image at the top? If so it's likely a capacitor in the vertical output section. If the lines appear all over the screen, it could be the CRT itself degrading.
When switched on it works "OK" but after a while if the screen is turned up at a decent brightness the top part becomes a bit brighter than the rest of the screen and will have some obvious scan lines then if you turn up brightness more the scan lines go but the top part remains brighter and with NO scanlines whatsoever, even less than normal.
Its really strange, and like in State of the Art demo when the screen goes bright white the sound also goes lowfor a second.
