Ok, the fact that the lines appear to be on every other scan line and the text looks fine over the top of the lines ( text with color has priority over sprites on the Galaga hardware ), I'd be looking at the the sprite RAM. Either eight 2147 or two 2148 depending on what's populated on the board. Also possible the 74LS296 at 5B, also make sure the resistor pack at RM5 isn't damaged or cracked.