S7 Chatter

I have a Super 7 that is well maintained and cared for. Just recently chatter has developed when doing just ordinary turning. The headstock was re-adjusted in accordance with the manual to no avail. Adjusting the main bearing to the point of stiffness does not improve the situation. New ball bearings have been fitted and properly adjusted although they probably weren’t needed. The mandrel seems to be in excellent condition and no obvious serious wear on the bronze bearing. Advice please.