It's been an interesting thread and there is still uncertainty over  which bit of the transmission is the culprit. When you get round to replacing the chain, cassette and chainring, it would be very revealing if you could change each bit one at a time and do a quick test after each change and observe what resolves the problem.
Good luck with the mechanics.
Keith
	
		
			
		
		
	
			
			Good luck with the mechanics.
Keith