Ford Ranger is haunted

I have a 2000 Ford Ranger, small v-6, automatic, no ac, windows, door locks, this has a heater and radio. This truck runs fine until I try to climb up the mountains (Colorado) and at app. fifty mph the engine starts sputtering and losing power. More than just an engine ping. When I run on flat ground while pulling a fifteen hundred pound load it runs fine with no sputtering or loss of power. Mechanics here are stumped.
Help

How many miles on your Ranger, and what is its maintenance history?

It sounds like the computer is having trouble adjusting the air-fuel mixture at high altitudes. I would start by checking the EGR valve and the knock sensor. When is the last time you installed a clean air filter?

Its a ford

Thanks badbearing,

I change the filter, oil, and oil filter every 5000 to 6000 miles

Possibly a weak engine (run a compression test) or a partially clogged catalytic converter. (can be checked with a vacuum gauge)

Maybe even both as one can influence the other.

I think the catalytic converter is a possibility. One of my colleagues did some mountain driving in the Appalachian mountains in Pennsylvania this past summer and experienced the same problem with his 2001 Pontiac Grand Prix. He had to have the catalytic converter replaced before he could get back home. I have a bad feeling that something caused the converter to becomer plugged and he will have problems again.

I have an app. with a shop and will ask them about these items.

Thanks for responding

Good schedule. Which reminds me…