Is your overflow plumbing able to handle full flow from your return pump ? 
I ask as I noticed during testing mine that there is a correlation between my sump level and my overflow level. Not just the actual levels, but the water level in the overflow affects drainage speed and so on. 
I am wondering; 
If your overflow can handle full flow maybe it will help you stabilize the whole setup ? 
Good luck 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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