I had the same problem with a 99 3.9L the radiator was the problem. I found this out after changeing the water pump, 3 thermastats, belts, hoses, checked timing, removing the exhaust for back pressure. This could have been avoided by checking the tempeture with a ir temp gun at different places on radiator. The lower temps means the radiator is plugged. This caused by the pink or extended anti freeze. Antifreeze needs to be changed yearly. Good luck:Ryobiguy
