I'd blace my bet on the deflection circuits (horizontal output transistor). I had similar problems with a more recent CRT, where the top right corner of the ima

I'd blace my bet on the deflection circuits (horizontal output transistor). I had similar problems with a more recent CRT, where the top right corner of the image started to shake frantically after a few minutes if you switched to high resolution modes. EDIT: It seems I didn't catch any of the details right... the author's display was probably not multisync and developed problems in the low resolution mode. Oh well.