The popular LumaFix is now also available for the C128! It is a tiny add on that goes into your C128. Your (hopefully socketed) graphic chip needs to be removed and placed back inside together with the LumaFix.

LumaFix helps to remove stripes, green/red lines or a checkered pattern that mostly appears on modern output devices via s-video. It has three toggles to adjust the video signal (and shield it from other signals that are also present), thereby improving the picture quality.