If the "waves" are side to side, moving up or down the screen, it may be electrolytic filter capacitors in aluminum cans. I don't recall you mentioning whether you replaced any of them. In particular I would look for a cap wired in series or parallel, in the main B+ circuit, right after the 5U4 rectifier or solid state rectifier if so equipped. A '56 would almost certainly be a tube rectifier.
Hard to tell from your description. Could also be Horizontal sync. Are you able to post pics yet?
Good Luck! Sounds like good progress so far. Nice find!
