I just bought a 2013 heckler used and found that it ghost shifts under heavy pedaling. I weight 230 so I am probably putting more pressure than average. But this only happens when I have smallest cog in front and largest and 2nd largest in back, fairly common gear ratios for a MTB.
My shifter cable housing runs all the way through to the derailleur and my LBS just changed the cassette and chain. I can physically see my rear wheel shift slightly off center when I apply a lot of pressure to my pedal while keeping the brakes on. I think this is the root cause, but don't know what to actually do about it. I don't think there's any damage to the rear triangle that I can see.
I'm also wondering if a bent hangar could play a role. I had a branch jam up in my hanger and it ripped it off and snapped the little metal hanger attachment. But I really think the derailment is coming from the top vs from the bottom. As in, I've caught the bike mid skip, and you can see the chain where it leaves the derailleur is in the correct spot, but above it the chain is 3-4 cogs below where it should be.
Any help is appreciated. This issue is sullying an otherwise amazing bike.