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One more possibility is that one of the rear derailer's tensioning springs has broken, or similarly that the chain is too long, or that the tensioning capacity of the rear derailer has been exceeded.
There needs to be a certain amount of tension in the chain in other words.

Note also that the upper "B"-pivot bushing is anchored at just one end inside of the knuckle, and that the attachment is known to fail under impact loading, leaving the pivot with an enormous amount of wobble/play.

Make sure also that the pulley bolts are secured tightly and that both pulleys turn freely.

A tight link in the chain is yet another common cause of chain skipping.
And check that the special connecting pin is not protruding excessively where it might contact an adjacent larger cog.

Lastly, be sure that the cassette is being effectively compressed by the lockring such that no cogs are loose against each other. Turn the wheel backwards and note whether the cogs are all parallel to each other, sometimes a spacer issue prevents the cogs from stacking up parallel. Also check that the last cog has it's splines indexed correctly on the freehub splines.