Mavic Cosmic Elite with 10 Speed Campagnolo

Hi all, first post and I can't find what I'm looking for anywhere.

I have been running a ICAN carbon rear wheel for a few years with a Capag 10s Centaur groupset. In search of a few more Strava seconds I put the aero front wheel on that had been sat in the garage for the same few years, only for the lightweight tyre bead to slide off in the rain leaving me picking gravel from my elbows and knees.

So I bought a set of Cosmic Cosmic Elite UST wheels (https://www.merlincycles.com/mavic-cosmic-elite-ust-clincher-road-wheels...) in the hope the made to measure tyres would never fall off the wheels. They only came with a shimano freehub, so I bought a Campag ED11 freehub and swapped it.

However, intermittantly the freehub is sticking. Most of the time it seems to spin freely. I used a light oil and it's clean. Off the bike I can spin the cassette and it'll rotate briefly after I let go. I'm note sure I'd expect it to be much freer. But on the bike, within 5 mins of riding I'll no freehub at all, intermittantly.

I notice the Campag freehub is 1-2mm shorted than the Shimano one it came with. I notice too there is a Campag spacer sold for the ED11, but it says it's need for 11s, I'm running 10. I do think it's to do with the compresion with the skewer tightened, though I can't figure out why the bearings would be compressed in this setup (they are all new).

The freehub came with a small washer, I have used that. I tried it without and the freehub stopped spinning completely.

I've refitted the old ICAN wheel and it still works, so I know i didn't bend the deraullier in the crash.

Anybody got any ideas? Merlin said they didn't know.

Cheers