Brewed a Belgian Dubbel yesterday and as noted earlier, performed a 90 minute mash. I then sparged with 180F water for about 15, mixed everything up real well, and got the 78% efficiency I got batch before last!
So, in my case, it seems as though the batch before last when I saw the jump, I sparged with a lower temperature water, which prolonged the conversion process and led to more extraction. Next time I brew, I'm going to perform another 90 minute mash to see if I got the same efficiency.
The confusing part to me, is that I've read that the conversion process should only take 45 minutes or so, which is why most people mash for 60. I wonder why in my case it's taking longer? If I can get consistent with my 90 minute mash, I'll drop it to 75, because I doubt the last 15 minutes of the 90 is really doing anything. This kinds of makes me wonder if I should check the calibration on my thermometer.