I've been having some efficiency issues lately, went through all the steps (adjusted the mill with feeler gaugues, made sure all my water volumes were spot on in Beersmith, etc) so I did one more experiment. Pre-heated my tun, the old orange rubbermaid cooler, and filled it with water. Started at 153.6F, and 70 minutes later was at 146.2F. That's 7.4F loss in 70 minutes, what gives?! I know a lot of people use these, did I get a dud or something? Can I attribute poor mash temp stability to trouble with efficiencies?