Thanks cristglenn for the response, but I think I may have more of a problem. Just after Baker on I-15 towing just under 3,500 pounds with and outside temp of 95 degrees, the A/C was shut off and I still have the same problem....saw a guy towing north on same highway with same truck just a 3500 DRW towing a loAded about 45ft enclosed fifth wheel and didnt seem to have to break speed due to the heat , his windows were up so I'm guessing his ac was on so I'm thinking there is a problem not to mention all the plp on here saying "I haul that road everyday and temp doesn't get above 220. Where else do I direct my attention ? The. Cooling system it's self ?