When this first started happening, it was on the interstate going through the mountains of New Mexico after about 4 hours of driving. The truck could no longer maintain 70 mph, because it started downshifting, revving up to approx. 5k RPM, and backfiring while slowing down. My first thought was a vacuum leak at the EGR hose where it goes into the throttle body elbow, since I had issues with that previously. I checked it, and noticed no issues, heard none of the noises I had noticed when it leaked previously, so I just wiggled it a little bit to make sure it was tight. After about 3 hours, started driving again and noticed no issues. I thought it was fixed, until a few hours later it started again, on level ground this time. At this point I thought it might be the fuel filter, so I replaced that. This resulted in no change. Meanwhile I had already put a couple of cans of SeaFoam through it, and that made no difference. Then I cleaned the MAF, which was a little dirty but nothing crazy. As I was plugging it back into the intake, it seemed like the rubber seal might be a little loose, perhaps leaking some air in there, so I used some electrical tape to seal it up a bit. No changes. I began to notice though that it seemed like you could push it a little harder when the engine was cooled for an hour or so, and even better the colder it got outside. It got to the point that if I could no longer maintain 65 MPH, I would pull off and turn the motor off for an hour with the hood up, and then I could do 75 for an hour or so. When I got home, I immediately replaced all ten coil packs. This made no difference. So I then replaced all ten plugs, and noticed that all of them were white like they had been running lean. I then hooked up my AutoEnginuity, and found that at anything above quarter throttle, short-term fuel trim immediately went straight to the max value on both banks. I then researched and found a lot of people have issues with intake manifold gaskets and the lower intake manifold. I replaced both, and also the gaskets on the throttle body and elbow. I have not driven it on the interstate since, but I have found the problem still persisting because when I punch it from a stop, it pulls fine to about 25 MPH, then quits pulling unless I pull it back to half throttle, then it can get up to 65 fine. I'm not driving it on the interstate until I fix this issue, as I would rather not melt something because I'm running chronically lean. I have checked my O2 sensor outputs, they are well within acceptable values. Readings coming from my MAF sensor are consistent. There is no catalytic converter. I have tried finding vacuum leak with carb cleaner to no avail.

My thoughts:
I don't think it's a fuel problem, because it gets worse the warmer the engine is. If it was fuel, I would expect that it would be consistent no matter the temp.
My next step will most likely be replacing hoses, pretty much all of them.

I love this truck but it is really making me have even greater respect for my 7.3, and if I can't figure this out soon, the V10 is gonna get parked until I can swap in another 7.3.

Please tell me if I'm missing anything. There are probably things I forgot to mention about characteristics of the problem and diagnostics I have already conducted, so just throw everything you got at me.

If you're still reading this, congratulations; I'm fairly certain your attention span is greater than 98% of adults.

 

No way of telling if its your fuel pump until you hook up a fuel pressure gauge and test it under load. Also check your PCV 90° elbow and see if its not cracked or collapse causing a vacuum leak.

 

TPS or maybe VSS need cleaning?

 

From the description given, it could be a slowly failing fuel pump that is giving up as demand and temps build but still provides adequate pressure/volume when cold/cooled down some. Check you fuel pressure.
These gas fuel pumps tend to die one of two ways, sudden death with absolutely no warning (like mine did) or very similar to what the OP has described, running OK when cold but laying down as temp or load increases.

 

Like everyone else has already said, check the fuel pressure under load

 

Alrighty, I will be checking fuel pressure under load as soon as I get a chance. Will update with results.

 

Also I did replace entire PCV assembly, I actually did that very shortly after I got the truck as it looked a bit worn out.

 

so did you check it under load, mine would fall flat at 3200 rpm's, fuel pressure major drop when under load at those rpm's. taped fuel pressure gauge to wind shield wiper arm and drove it, yep bad 17 lbs. new pump. like new again..