put your hand on the job and gently grip it as you cut

Sounds like a thrust bearing/bearing problem in the spindle . IF it cuts smooth running from spindle to tailstock then the tailstock is taking up the thrust loads and cutting smooth.

Just a thought, (;-) TP

Another thought, the "pitch" of the banding might be a direct reflection of the pitch of the Z ballscrew. (5mm is common)
Could it possibly be something there, or maybe the screws support/thrust bearings ?
Maybe just a coincidence.
Russ

Hi, I cut some ally bar stock today placed in the chuck and only 50mm protuding, I did a couple of 40mm passes and I can see the same banding although very faint but it's there.

I adjusted the preload on the headstock a couple of years ago because of some chatter I couldnt get rid of, the
motor used to spin up to 2000 rpm but since the extra adjustment I can only get 1800 rpm before the vfd cuts out.

It's something I need to look at again but I'm just thinking I know the headstock is tight.

This brings me to thinking (worrying) and  as Russ mentioned about the ballscrew. But how will that cause a problem?

The carriage runs on a raised V so cannot twist.

 

One more question, is X a servo or stepper ?
If it's a servo, could it possibly be "hunting" ? Could require tuning ?
Just a shot.

This brings me to thinking (worrying) and  as Russ mentioned about the ballscrew. But how will that cause a problem?

Not sure about the screw itself, but depending on the support bearing arrangement, if paired as illustrated, the loaded bearing in one direction may be perfect and the opposing one might be failing when the direction is reversed. A very careful backlash investigation may reveal a problem here.
Just a possibility,
Russ