I've got an '01 F-150 with the 5.4L non-supercharged motor with 128k miles. For the last couple years, it's developed this nasty habit of infrequently coughing, sputtering, and surging, when the ambient temperature is high (like, over 90*f) and it's above 1,000-1,500ft elevation. Did the same thing last night. I was driving through the lovely hamlet of Grapevine, CA (elevation 1400 ft) and it was about 107*f outside. I had driven there from Fresno and experienced no issues whatsoever up to that point. I got off the highway and slowed to about 15 mph, and my truck started wanting to die or surge, or both. I parked for around 30 minutes, started it back up, and it was fine... Or so I thought. I continued on my journey and drove up to about 3,000ft elevation, and the cough, sputter, stall, and surge was back. I got off the highway, turned around and headed back down the hill, letting the truck coast back to the valley floor. Drove the remaining 145 miles back home and the truck performed flawlessly (even managed 16+mpg!) I took it in for the same problem a couple years ago, and they couldn't identify anything other than a "lazy O2 sensor" which I had replaced. The fuel filter, and fuel pump are both relatively new (I only put @ 5k miles on the truck a year.) The "Check Engine Light" has never come on throughout any of its episodes, and it shows no codes on a scanner.

Plugs are newer Motorcraft gapped to spec and torqued to spec. I replaced the MAF sensor about a year ago, replaced all COP's at the same time. I'm thinking maybe a vacuum leak somewhere? But the altitude thing is really throwing me for a loop. Any suggestions? I'm all ears.

 

I'd throw a couple of bottles of Gumout for high mileage engines in the gas tank and run it through. That will clean up the injectors and intake valves.

 

The only part that reacts to temperature change on the outside is the Intake Air temperature sensor (IAC) in the intake hose tract.
I would check the sensor for being close to out of tolerance at high air temperatures.
This sensor has a lot of 'sey' in system operation for A/F control.
Use a Scanner to look at the air temperature and see if it relates to your issue.
As the elevation get higher the air density gets less and less (leaner) plus the temperature of the intake tract can get higher due to engine load.
All these can add up to an issue.
Also there might be a faulty coil causing the grief if it has shorted turns that would lower its voltage output under lean conditions causing misfire.
With good understanding of how to use a Scanner, you should be able to see the cause.
This is not a case of reading or setting a code but looking at data since nothing has actually failed. Its a matter of tolerances.
Good luck.