If you're batch sparging you don't need to worry about channeling. Mix the crap out of it and then let it run as quickly as it will let you.
Are you sure you're hitting your temps? When I first made my mash tun I thought I was hitting temps but couldn't get my og up high enoug. I started monitoring it a little more closely and my numbers got better. I try to add water to my cooler at 175+ and then I close the lid for 15 mins or so to warm the cooler up. Next. I open it up and stir the water like mad to get to my mash in temp, add my grain, and stir a little more. I aim for a degree or 2 high. If I'm higher than that I can stir a little more to lose some heat.