This is occuring on the 2nd-4th sprockets on my cassette. The entire drivetrain is new (SRAM cassette & chain, SLX rings & BB on a Helius CC with a single BB spacer on the LH crank side & 2 on the right). The rear mech is XTR.
Exactly the same thing had been happening with my old drivetrain, but I had assumed that it was worn components. Clearly I was wrong. What else could be the cause – the mech hanger? It's not obviously bent, & wouldn't it affect shifting over the entire cassette?

Just throw this out there.. are you running the correct shifter for your gears? I was running a 9 speed shifter on an 8 speed cassette for a while and experienced a very similar problem (it was desperate times!).

It's all 9 speed & used to work perfectly.

no worries, sorry I can't help mate

I'll take a SWAG (Scientific Wild-Arsed Guess) at this and say…gear cables. Make sure they're clean and lubed and maybe it'll solve your problems.

How olds the chain?

yeh, your kit's all ruined. Tenner for the rear mech ?

or clean/replace your cable(s) is favourite – could be the hanger but shitty cable is most likely

even so, I think a tenner's fair 🙂

I cleaned the cable……………& it all works!
The old kit will be put back on to see if I have just wasted a load of cash.

"Stiff link ??"

This will make the chain skip on the small cogs only

Had some lovely suggestion and then read that you'd claened the cables and solved it.