definitely look at your chroma encoder circuit (CXA1645 near AV jack). Likely your 220uf output caps for CVBS Y, or C are going bad.
If you have an RGB or SCART cable its also a great way to rule out other issues. its an issue with your color burst (NTSC / PAL only, sometimes its also a bad clock pulse going to the encoder. Thats a lot less common tho.