I've had a bear of a time tonight.
Yesterday, I cleaned my winter commuter bike up.
This involved chain off, cassette off, cranks off, and all put through the cleaner as per normal.
Re-assembly as per normal.
Checked the chain wear with the park tool, and decided it was time for a new chain.
Put new KMC 10-spd chain on.
This morning: un-ridable. Chain skipping under even the most modest loads.
I'm not talking honking on hills; just modest flats.
Hmm, cassette might be a bit old, and skipping on the new chain.
But that's usually only on the most-worn sprockets. Here, it was on *all* sprockets, even the 12 and 25, which see no use!
Swap to a nearly-new cassette off of my disk TT wheel: no better.
Chainwheel?
Same on small, middle, and big.
Swap middle chainring for a spare I found: no better.
Freehub slipping?
Swap rear wheel: no better.
Replace chain with original: all fixed.
Looks like a bad new chain.
Or something else?