Use an insulated implement, like the plastic handle of a screw driver to tap on the bottom of the analog board with the machine running and see if the image changes. If it does, you have bad solder joints and potentially bad potentiometers that need addressing. Lots of weird display issues can be caused by bad solder joints.
I would recommend redoing every solder joint on the analog board, because there can be microscopic cracks and separations you can't see with the naked eye, or even under magnification.
For the potentiometers, I'd recommend marking their position with a sharpie, then hosing them down in Deoxit Gold G5 and wiping them back and forth several times. You may see oxidized black carbon come out of the wipers, clean that off after you're done wiping and set the potentiometer back to as close to its original position as possible.
As you probably know, these machines ran smoking hot because of Jobs' insistence that his precious had no fans or vents. So most of the compact mac line with the exception of the SE, SE/30 and Color Classic series don't have fans, or the appropriate ventilation The tiny slots under the handle and on the top corners are nowhere near enough. You may consider modifying the case to add a fan if you're not too concerned about being a purist and leaving the machine unaltered externally.
