STUMPED! 03 Ram 1500 5.7 Hemi 4wd overheating

My '03 Ram 1500 5.7 Hemi 4wd keeps overheating. Replaced the water pump, thermostat, hoses, radiator, radiator cap, engine oil pressure sensor, ECT sensor and I'm about to put in a new fan clutch. Truck is still overheating. I've flushed the system twice, including the heater core. I pressure tested the cooling system after all the repairs and found that the new radiator cap was defective. I exchanged it and tested the new one before leaving the parts store. When it overheats, it's acting like the thermostat isn't opening and the heater blows cool air. The upper hose gets too hot to touch and the lower is cool. It's like there's no flow. I pulled out the new thermostat and tested it. It works like it should. I was thinking heater core, but there's no moisture inside the cab, no coolant smell and no dampness on passenger floor. I've burped the system multiple times with a no spill funnel with the front end raised for a good 30 minutes each time. It seems like air keeps getting back into it somehow. I suspected maybe a blown head gasket so I did a combustion leak test twice and both came back good. Through all these changes, I found that the AC compressor is going out, so I bypassed the AC pulley and idler pulley with a shorter serpentine belt for now. The heater blows hot initially sometimes and everything seems normal until the temperature hits about the middle of the gauge. Then the air gets cool and the truck gets hot. I shut it off right before it hits the redline. The upper hose is too hot to touch when this happens and the lower hose is barely warm. A couple of times, I've thrown it in neutral and quickly hit and released the accelerator one time to rev it to 3500-4000 rpm and the heater starts blowing hot again and the temp gauge goes back to the normal range. Another thing that's really got me stumped is that after I've burped the system and then topped off the radiator and reservoir, within an hour or 2, the reservoir is almost completely empty, yet there's no fluid on the ground and I can't see or find any leaks anywhere. Where is the coolant going? The volume it's losing is far more than what could evaporate over that short amount of time. There's no indication of coolant in the oil or oil in the coolant. Initially, the check engine light came on and the codes I pulled were P2181-Cooling System Performance and P0522-Oil Pressure Sensor/Switch Low Voltage. The new oil pressure sensor took care of both of those. The next time the check engine light came on, it threw codes
P0442 -small leak in EVAP system and P2181-Cooling System Performance. The ECT sensor got rid of both of those. It's currently not throwing any codes and the check engine light hasn't come back on. I've found several threads on various forums with the same or similar issues, but have yet to find one that figured out the problem. Any suggestions? I don't know what else it could be. Thanks!!

Wow ok you did alot So the cooling system is rather simple on these trucks It sounds like you still have a ton of air in the system. They can be a slight pain in the **** to burp the system on these let the truck run for quite a while up to operating temp with cap off rev engine up and down aswell as push on upper rad hose you will see bubbles once at operating temp only way i have done it that works. You also may have a blockage somehere but unless someone used stop leak at some point its not likely. You say you see no leak anywhere and you did the headgasket test I would let the truck sit a day or so to let things seprate and loosen oil drain plug a little just confirm no coolant/water comes out first. Also by the way the heatercore alone wont make the truck overheat I have also had many bad brand new tstats out the box i know you said you tested it but i would confirm to make sure its good again and that you didnt put it in backards that happens to ask me how i know Lmfaoo.

I was going to second the T-Stat direction, and running it with the radiator cap off to watch the flow.

Had a similar issue on my Jeep, the new thermostat was defective.
Bought an OEM new one and all problems were gone.
I tested the other one by putting it into some boiling water and watched if it worked.

On my 06, I for sure take my time to fill the radiator and burp it as much and as often as I can. Like jws123 said, squeezing the upper hose often. Ever time I try to rush the job, I get the same overheating problems with the dash dinging to check gauges. Only twice I’ve run into a defective t-stat. I’ve learned to test them before I spend all of the time and wasted sealant to have to take it back apart.

When you replaced the serpentine belt did you happen put the new one on so that it is turning the water pump the wrong direction? My son did that when he used a different belt to bypass the a/c unit one night in the dark. It was the only way that belt seemed to fit the pulleys.

Id also daresay pull a vacuum to the radiator and see if you can pull some air bubbles out. Not saying low pressure vacuum, just a regular shop-vac kinda vacuum and pull as much out of the line that you can. There could potentially be debris in the line that can work its way out if thats the case. Pull from both the top and bottom of the radiator, but open both the cap and the lower weep hole. Also would behoove you to pressure test the system for leaks if you havent already. But you should also check the radiator fins for damage. What type of thermostat did you install? Some ppl install a 180 degree t-stat to help alleviate high temps. Heater core clogged? That would be my guess. If I tear anything apart, that would be the first thing I did.

I remembered something from my 2004 Ram...it was always low on coolant, but it never left a trail. I found a crack on the lower radiator hose flange that sealed perfectly when not under load and would just spray out above highway rpm. Those old radiators are made from plastic for the most part...and plastic is susceptible to more wear and tear than normal aluminum ones. Not saying thats what the problem is here, but might be worth a look too. It took me a month to find that problem.

The upper hose being Hot, and the lower hose cool tells me the Radiator is doing its job, I would get a new OEM thermostat and replace it leave the radiator cap off while squeezing the upper hose, to get the air out of the system, also what condition is your lower hose in, is there a spring in it, if not it could suck flat feeding the water pump inlet, check that by revving the engine to see if its good, hope you get it sorted out.

Another way to "burp" the system is take out the temp sensor and fill until you see the coolant and reinstall.