Lathe cutting fault finding strange thing happening

[Nemo1966]

I have a Denford Easiturn and recently when taking a cut near the chuck instead of the cut being flat... it is wavey. The ballscrew is 25.. diameter x 5mm pitch and the cut mimicks the ballscrew e.g. it goes in for 5mm and the out for 5mm (or it get larger in diameter for 5mm then smaller in diamter for 5mm).




Has anyone any idea what causes this? Worn ballscrew? Worn bearings?

thanks