Rear derailleur skips under load after a new chain. Worn cassette?

Classic symptom of a worn cassette after a late chain swap. The new chain does not sit in the hooked teeth. Before you buy anything, check the limit screws and cable tension, just to rule them out.

Put the bike in the smallest cog and look at the teeth: shark fins mean it is done. At 0.75 with 6,000 km, the cassette has almost certainly worn to the old chain. Replace it, and next time swap the chain at 0.5 and one cassette will outlast two or three chains.

swap the chain at 0.5

This. I keep a checker in the saddle bag and test it on the first of every month.

Same thing happened to me last spring. New cassette and the problem vanished the same evening.

If the chainrings are original too, have a look at them while you are in there. Worn rings are rarer but they do happen.

A 12 to 25 tooth range cassette is cheap if you only ride on the flat. Worth a look if you are replacing anyway.

Tip for fitting: grease the lockring threads lightly and use a torque wrench. 40 Nm is typical, but check the print on the lockring.

You will want a chain whip and a lockring tool. Fix-up night has both if you do not want to buy them.

Could also be a stiff link from the new chain. Backpedal slowly and watch the chain go through the rear mech.

Did you shorten the new chain to the same length as the old one? Count the links, it catches people out.

Worth checking the derailleur hanger too. A small bend makes skipping worse in the smallest cogs.

I measure my chains with a ruler as well as the checker. Twelve links should be 12 inches exactly.

Jonas is right about the teeth. Take a photo of the 11 tooth cog next to a new one and the difference is obvious.

For what it is worth, a 105 cassette lasted me about 12,000 km with three chains.

If you ride through winter, clean the drivetrain more often. Grit is what wears the teeth.

Bring it along on Thursday and we can check everything in ten minutes.

The skipping under load only, never when spinning easily, is the giveaway. Cassette.

I had the same with an old freehub body that had notches in it. Unlikely, but look once the cassette is off.

New cassette and new chain together is the only way to be sure. Old rings are usually fine.

Top tip: write the date and the chain measurement on a bit of tape inside the frame. Helps you remember.