Cell voltages are a very undependable measure of battery usage, partially because the mid part of the discharge curve is very flat, and partially because the cell voltage varies with temperature, current draw, and just about everything else. You want to look at the amount of capacity that's returned to the cell during charge. I assume your charger reports how much it puts back into the cell -- how much is it?
If you take that "capacity returned" number and multiply it by 1.25 to get a cell capacity, then those cells should end up being discharged by 80% at the end of each flight. That's the smallest cell size that you want to use.
I just ran your reported numbers through a spreadsheet that I have, that distills the numbers from the various planes in the "list your setup" thread here. It doesn't pencil out. At 42 ounces and three cells, I get a prescription for 3500mAh cells (assuming a 75% charge use at each flight). I'm also predicting that you'd need 300W average power and 400W peak, which would leave you with a pretty hot motor. Also, for the Kv of that motor with three cells, you want to launch at something more like 8000 RPM (assuming you're using an ESC with 'helicopter mode' governing).
Either you're managing with a lot less power than the rest of the folks on the list, or you're really flogging your motor and especially your batteries. And if I'm not mistaken about the launch RPM, you're also seeing some sagging toward the end of the flight.