H end stop set up right? With the cable not clamped down, and using your left hand, pushng and letting go of the rear mech whilst pedalling does it fall back to smallest, and push up to largest, and neither case go to far and derail onto spokes or between frame?
That's bang on
Slipping as in the chain slips on the cassette teeth or its trying to go up/down a cog and isn't getting there? Always in a certain gear or across the range?
Definitely the chain/cassette and not the freehub slipping?
It feels like both, slipping on the cassette teeth but as though it wants to go down a gear but doesn't. The Freehub is solid.
Check if the mech hanger isn't bent. At at H end stop position, it's a straight line between the sprocket and jockey wheels.
This needs checking because I did go down on my right side on black ice on November 7th BUT this problem didn't start until about 4 weeks ago!