Chain not worn constantly slipping

[Ming the Merciless]

Virtually yes. A couple of skips between middle sprockets then everything.

Have you taken wheel out, then grab cassette firmly and see if you can move it in or out or side to side? Under tension implies either free hub bearing issue or loose cassette. Might be free gub given the way it suddenly spread.

How many pedal rotations between skips?

 

[Big John]

If you've got an old/spare wheel you could try the cassette on that, it would at least tell if it's a freehub issue or not.

This. Assuming that the test wheel has got a decent freehub in it.

 

[Fifelad]

If you've got an old/spare wheel you could try the cassette on that, it would at least tell if it's a freehub

 

[Fifelad]

I did that. Freehub issue confirmed. I took to the wheel to the LBS A ratchet system rather than pawls. New ratchets ordered they were gunked up so LBS cleaned them but on assembly still not right, looks like they were worn.