1997 Ford Explorer cooling system

97 exploder 4.0L SOHC. Coolant fills the reservoir and makes it over flow. Replaced the rad and thermostat, flushed and back flushed the heater core. When vehicle is first started in the am and is cold, it idles like low. Then when the truck shows its warming up it still blow cold air with the heat on. It finally starts to blow warm air and while driving the temp gauge rises till almost to the overheat line, then it drops to half way and stays there. I've pulled over and checked the heater core inlet out and outlet tubes and they are both close in temp. The upper rad hose is hot but the lower rad hose isn't even warm (cold to touch). Done multiple coolant pressure tests and no signs of pressure drop. There is a fair amount of water dripping out the tailpipe though. No coolant in the oil. The only thing I think it could be is the water passages in the heads are plugged and not allowing the coolant to properly flow thru the entire system hence the hot upper rad hose and the cold lower rad hose. Am I correct or is there something I am missing? Thanks in advance