8 gallon Coleman mash temp variations

[Rohlk]

I mash in an old Coleman cooler(8 gallons) and I've noticed huge temp differences in areas of the cooler. Corners are different from the center, bottom different from the top. I stir like a ***** and still variances. Anybody else have this problem? It can differ as much as 6 degrees around the cooler. WTF? I preheat, FYI. I'm thinking I might turn my old 8 gallon kettle to a mash tun. Is it the geometry of the cooler? That's my guess.