I've been brewing AG for well over 3 years now and have probably 25 or 30 brews under my belt, so I'm pretty comfortable with my process and have a good system down.
Today I brewed the biggest beer I've made to date, it was a Hunahpu's inspired Imperial Stout. The total grain bill on the 5 gallon batch was a hair over 20 lbs including the rice hulls and I mash in a 10 gallon square cooler. I have a digital probe thermometer installed at the bottom of the cooler, about 2 inches above the bottom, and then I also measure temp at the top of the cooler . Today's brew dang near filled the cooler to capacity. The problem I had was I COULD NOT for the life of me get my temps to equilibrate throughout the entire cooler... I stirred and stirred and stirred and stirred, and the reading at the bottom of the cooler was right at 160 and the top was right at 151. My target temp was 155, so after about 10-15 minutes of constant stirring and no luck I just said screw it and figured I would have a good mix of high- and low-fermentable sugars. I even drew about 2 gallons of liquid out and added it to the top and it didn't hardly do anything to the temps.
Any suggestions on how to get a more even temperature distribution? I know that I need to get a better mash paddle, that will probably help some, right now I'm just using a small cheap plastic jobby that doesn't move much grist. My efficienies are usually in the 75-80% range, however today I was low, came in about 65%. My target OG was 1.104 and I 'only' hit 1.086 (according to my hydrometer, my refractometer read 22.2 which is 1.093).