New chain slipping on new cassette

[PomBstard]

Have just fitted a new SRAM PC970 chain and PC990 cassette onto my bike and the chain is slipping on one particular gear. Its not at one end of the cassette, but the seventh from the wheel. Works fine in the other eight gears, just slips on this one.

Doesn't feel or sound like the chains not lined up, but properly skipping a link. Any clues? Have done minor tinkering with the rear mech, to no avail so far.

[Vladimir]

Might be worth refitting the cassette. That cog may not be quite seated correctly (I have the same one but with the 991 chain). Have you tried all three chain rings just to make sure it's not a chain angle issue?

And lastly, is your hub newish? Once they (the pawls) go, the cassette slips constantly.

[PomBstard]

Er, move along, nothing to see here. Problem? What problem?

Refitted cassette and it was subtle, though still enough for me to exclaim that I was indeed a daft tt. Most of the gears on the cassette are attached to a spacer, except for 7 and I had got that mixed with the spacer next to the hub - just slightly too thin to allow the chain to run through the gear properly.

Refitted, runs lovely, watch and learn moment for others...