Typically I'd get about 82% mash efficency with fly and I only got 70% this time. My question is where am I messing up? I did two equal run offs through Beersmith and the second one was a 180 degree infusion which resulted in a 166 mash. I stirred it gently and let it sit for about 10 minutes before running it into the mashtun. Any thoughts?
Given that you get higher efficiency with fly sparging, I think the major mash parameters (crush, pH, temperature) are probably in line and your losses are coming in the lauter. My first suspect would be incomplete draining of first runnings - did you verify that the runnings were actually equal in volume? Another possibility is that if you are saving significant time over fly sparging, you aren't allowing for complete conversion that was previously happening during lautering. I always verify the gravity of my first runnings before I start running off.
If you plug your numbers into this calculator you can see how close you were to the theoretical efficiency: http://seanterrill.com/batchsparge/