The chain was jumping teeth from the 4 smaller sprockets when pushing.
Something you would definitely feel/hear. But it was still fine on the 7 bigger sprockets.
Putting a new chain didn't improve.
Visually, the cassette still looked good.
I contacted the seller( bike24.com ) send them a picture, and tried to get some warranty.
They told me it was worn out. And that many riders had the same issue, depending on how they cared about it...
I explained them I was cleaning/lubing the chain after each ride, and changed 3 times the chain. To no avail.
When I got the new cassette, I just swapped.
With the same chain, derailleur settings, etc. The problem disappeared immediately.
You are very lucky to have that mileage, with a single chain and cassette.
May be it is the mud. Or I may do something very wrong.
My upgrade was expensive indeed. But the business was good this year.
If my 256 was still fine, I could go for the Epic too.
Well... after a road bike, and may be a trial bike