How is a black and white picture? (Turn down the color control).
If B/W picture is good, then the problem is somewhere back in the chroma decoder circuitry. Check the video input to the CRT video driver board and signals on that board. If B/W picture is also bad, then you can compare red and green signals to determine where they are becoming different. The red lines in your description sounds like the red video output circuit is drifting and messing up the background level, blanking, screen, or other setting. Could be a capacitor or other component.
