Running 105 everything on my roadie, and it has developed an annoying habit of the chain slipping over the cassette. It’s not the indexing etc as the chain stays on the same sprocket, but seems to ride up the teeth then slip over a few teeth before settling down again. Happens maybe once every 20 pedal strokes

Tried changing the chain, cassette, front chain rings and the rear mech to no avail- any ideas!? Am I missing something really obvious (distinct possibility)?

Play in the hub/freewheel.

Cassette not sitting perfectly square on the freehub.

Upload a video.

Stiff link?
Bent link?
Old cables that need replacing?
Bent mech hanger?
Loose mech manger

Chain is fine (almost new)
Cables have also been replaced in the last month or so

Mech hanger is a possibility
Hadn’t thought about the free hub/cassette seating etc. off to the garage I go!!

How do you know the chain is riding over the cogs?
Is it the freehub not correctly engaging and spinning under load?

Chain is fine (almost new)

Not to say it doesn’t have a bent/stiff link though.

Stiff/bent link would be my first bet too. Regardless of how new it is.

Are 10 speed chains ‘directional’?

I think the jockey wheels are too so might be worth confirming they’re running the right way.

b screw on the mech (I have seen LBS’s mess up or forget to adjust that)

bent hanger, frame, etc.

assume the replaced cassette was new too? and the “right” chain type for the cassette.

This happened to me. It was a loose cassette lockring allowing the indiviual sprockets to wobble a bit.

About the last thing I thought to check.

Shimano chains are directional.
Lettering should be facing you.

Similar issue here before but when I had swapped cassette over from wheel to wheel, I forgot the spacer closest to the wheel (on a 105 cassette) so the whole cassette moved as and when it wanted to.

Resolved after much head scratching! Seems I was in a rush when I changed the chain, and had out on a 9sp quick link rather than a 10sp.

Thanks for all the suggestions- whoever said the chain was the closest!

Can’t believe the LBS didn’t notice that.

had out on a 9sp quick link rather than a 10sp.

Hmmmm….. that shouldn’t be an issue though. I’ve run loads of 10sp chains with a 9sp quick link. Maybe that one had worn down quite a bit too, so it gave some good chain stretch in those plates.