Does physically holding the tube change anything? I had a unit a few years ago that was self oscillating, and putting tube dampers over the tubes helped a lot.
That being said lead dress helps a lot, which is why I like the ribbon cable. I obviously won't say it's required and I understand some people don't like the look or are uncomfortable running it right behind the tube, but short, straight wire runs with the wires perfectly in parallel and a pair of DC (ground and V+) signals separating the input and output seems to help for stability.