96 Dodge ram 2500 overheating and blowing cold

[hx15018]

Hi I have a 96 dodge ram 5.9 that keeps overheating gradually and blows hot and cold air in the cab. When I first drive it stays below normal temp level and blows hot and cold in to the cab and then after about 40km of highway driving the temp starts to climb and then overheats. I have replaced both head gaskets aswell as having the heads pressure tested and refaced, I then found one of the heads was cracked so replaced it with a new one. I also replaced the water pump and thermostat and flushed the whole system but I am still having this problem I can sometimes keep the temp low by driving below 80km per hour could this be a possible air lock or blocked rad any ideas would be most appreciated???

[rockwood84]

the rad could be gunked up. is then fan turning ,no slipping. the fan clutch could be bad. see if the fan will wobble if you move it back and forth.could be the t-stat if it was made in mexico as these will not open in boiling hot water.