Been watching this for a while & what about an air bubble? Bad thermostat? Just because it's new does not mean it works right, especially with thermostats. If the system wasn't drawn down or burped properly when the cooling system was refilled it could have an air pocket.
I've had a stat replaced twice - once was the stat that was the issue & the other time there was an air pocket. They even used a cooling system vacuum to draw air out & it still had a pocket. We eventually lifted the PS side of the vehicle & revved it to 1000 or so for a few mins at operating temp, to get the air out. Old School wins again lol.

PS as for towing, my '02 has a rated 5000lbs towing capacity & I think 600lb tongue weight, class III hitch. Not sure what an '04 has but it should be equal or close to. Google it & see.

 

I'm beginning to sniff a trend regarding thermostats. and this is my thought on them:
From the eighties till now, I bet I've gone through 15-20 cars most or all over 100kmiles, and had maybe ONE fail.
Now.. Bravadiva is running low ect {with all the folks have this same same problem.. looks like the overheated stay open feature also ratchets the pellet} and Tbird {with probably AZ cheapo in it} is sticky - air bubble ruled out.
- BTW air bubbles usually manifest by fluctuating temps/ slow warm then overrun at first open and often late heater delivery.

T Stats aint a commodity. I'm going back to the brand that MEANS thermostat.. Stant. If you cant get a Stant brand T stat that works right, then they are ALL a roll of the dice.
http://www.stant.com//Consumer-Produ...t--Thermostat/

http://www.stant.com/modules/partLoc...m.cfm?id=45119

Under $7.00 at Rockauto and Summit. T'd provisionally recommend these over AC Delco which is OEM

 

the more expensive napa t-stat is made by stant(says it right on the box) that's what i've got in mine, works great ... now

 

I would lean toward fan clutch myself. Pop the hood with the engine running. Get to a point where you can both see the fan and touch the key. Shut the truck off. If the fan spins for more than a few seconds, the clutch is bad. A bad fan clutch is worse than no fan at all.

 

I changed my tstat to a stant like many suggested with no change. All other parts are new as well and two shops say there is no leak in the radiator. My question now is the sending unit but on the ACdelco website there are 2 coolant temp sensors with different part numbers. I've located the/a sensor on the drivers side block near a plug wire. Is that the only one? And if that isn't the problem, whats left? I still cant believe that an 8y/o truck with 105k cant tow half its recommended weight.

 

Sounds to me like you have a cooling system problem(s). I've towed more over less miles in hotter temps with no problems. Might want to pull the radiator and have it acid cleaned. Also, as has been mentioned, the clutch on the fan.

Though my bet is on the radiator.

 

My moms 04 when her cluch fan went it got stuck on full all time that I found out is default at the dealer
What it looks to me is you did everything but replace your rad cap if it dont work proper you truck can get hot bye not letting fluid out or not takeing it in or even letting out too much

even with the flushing you rad could still be cloged it may be time for a new one

I tow just about anything I can with my 2000 zr2 in drive if the trans needs a lower geer it will pick it im sure I hav over loaded it a few times on a couple of 6 hour trips never had a over heat problem ran same temp hot or cold out with ac or heat on yes temp gage works fine

 

a couple stupid things you can check that i didnt see mentioned was look at the front of your radiator. not at the front of the ac condenser, look down at the radiator itself and make sure its clear. last summer we had about 5 cars come in due to over heating and the radiator is just jampacked clogged up with cottonwoods and other plant life lol. just stick the garden hose down there and blow it out, compressed air really doesnt seem to work.

also check your fan shroud make sure both upper and lower pieces are there and that your fan is actually pulling air through the radiator.

 

Thanks for all the suggestions guys and I've looked into them all. Cap and fan clutch are new as well and the radi and shrouds are clear. I mean I literally have replaced every part of the cooling system except the radiator, which I may end up doing just for the hell of it. I went ahead and burped it again today just to see. No road trips planned soon but I may just hook up a trailer and see what happens.

 

You said you had the system flushed
some places run a some stupid flush stuff that "breaks down the stuff in the system"
when I flush I flush with water
I pull the thermostat and the hoses and flush out the Block Backwards bye its self
I flush the rad Bye its self backwards
The heater core also Backwards bye its self

I will tell you why
I used some prestone flush kit 1 time
you cut the heater hose and put this t-fitting in That you can screw a hose on to and then cap later
I added the flush crap and did what ever the stuff said to do and then ran the hose till it came out clear
still had problems
I asked my dad about it (I was like 17) and he he told me to pull all the hoses and do what he said to do

you should of seen the mud that came out of the block the rad
what was going on was the flush wasnt doing crap and when I was back flushing the system all it was doing was getting stuck and cloging up the rad
till I ran it again and it all broke up went threw the system and I was back to where i started
It takes time to do but well worth the time
O yea 1 other thing make sure you have a 50/50 mix in the system its not only anti freez it is also a coolent good luck