I think this is freehub related. If the inner freehub bearing fails the freehub will "tilt" on the axle. This will cause the cassette to "wobble" as it rotates. It will cause people to think it's "skipping" when it's really "ghost shifting" as the cogs move in and out of alignment with the derailleur.

You cannot feel if the bearing has failed by pushing/pulling on the cassette. Remove the wheel and cassette. Rotate the freehub. If it rotates concentric to the axle, then the bearing has failed. You will need to disassemble the hub and replace the bearing. Depending on how long the damage has existed and how extensive it is you may need to replace the internal bearing spacers.

I am going through this exact issue with a Sun-Ringle Dirty Flea hub at the moment.