So, have a new SRAM 990 cassette, fitted it and had some play. No problem i thought, popped in a spacer, still wobbles

tried a thicker spacer but couldn’t thread the lock ring on (in other words, can’t get another spacer in)

Got me stumped, anyone had similar?

Its the freehub?

They all wobble a bit ime

Assuming the cassette & hub are compatible check the freehub – it needs about 60lbs of force applied at right angles to the end of a 6″ hex key which is tight – really tight.

If it’s Mavic crossmax wheels, it could be the plastic bushing that’s worn, allowing the freehub and the cassette to wobble together. Fairly common. Easyish to fit with either a new bushing or bearing conversion.