If you look at the fourth image in the photo album where the bottom right 'button' is active then the artifact appears on the very top line of the scan, presumably displayed on the following field. The artifact is also triggered by the symbol for the rotary control knob but only where the approximation to the circle extends to the very last pixel on the scan line.
This suggests a way to reliably look at the video signal where it is displaying the artifact. You can trigger from FSYNC or VSYNC, it would be interesting to see if the artifact was on all four fields or just one. You then use delayed sweep to select the first scan line and depending on the polarity and timing of the vertical sync pulses you may need to change the delay for the different trigger options. So now you can trace the video signal from the RAMDAC through to the output of the video amplifier at the clipping diodes CR560, CR568. The video amplifier has a lot of high frequency peaking including a few inductive elements so there is plenty of opportunity for it to ring because of a fault or an incorrect video drive signal.
Good hunting!
Regards,
Roger
