The fluids (transmission, radiator, oil, steering, etc.) were all flushed and changed about 10,000 miles ago at the 150K overhaul. Everything has been checked and topped off (if needed) recently, and the local Dodge dealership said there's no mechanical problems with the radiator, the thermostat, or the coolant pump(s). I really think it's just a matter of being at/over the towing limit combined with the extra stress of climbing a long and very steep hill - Parley's is a 12.5 mile climb at 8% grade. I was bored last weekend so I decided to test that theory. With the trailer and its cargo deck loaded up with all of the stuff we normally take camping, plus coolers weighed down with a bunch of water bottles simulating a weekend of food inside the Durango, but WITHOUT my wife and kids in the car, I pulled the trailer up and over Parley's Summit without any significant overheating. The engine did get warm, but not enough to cause a problem. So I've decided that there are several options: Get a new tow vehicle. (Not in the budget any time soon.) Pack a lot less stuff. (Have you tried telling four women - including 3 teenagers - to pack less stuff? ) Go camping without the kids and their stuff. (VERY tempting sometimes!) Avoid steep hills as much as possible. (This will be the default choice most of the time.) As a secondary confirmation of the theory, we went camping about 6 hours away a few weeks ago, and were actually carrying about 400-500 pounds more than normal on that trip with a full 35 gallon water tank plus a generator and a big tent that my Dad couldn't fit in his car. I stayed around 65-70mph for most of the drive except when we passed through small towns with lower speed limits. We didn't have any overheating during that drive except for one somewhat steep spot near the Utah/Idaho border called Rattlesnake Pass where the temperature did start to climb a bit but didn't force me to pull over since it wasn't nearly as steep or as long of a climb as Parley's Summit. The engine didn't really warm up at all on the way back home on the same route carrying the same gear but with the water tank empty and most of the food gone.