03 4.6l crew cab power loss up hilll

[Sarhan]

Background:
I own a 2003 GMC Sonoma crew cab 4.3l vortec v6. It has 130xxx miles on it.

Issue:
So i drive up a long steep hill usually with the gas pedal to the floor. When I hit about 60mph and the rpms are around 4k the truck stops accelerating, starts decelerating, the engine starts making a clunking noise and then the engine will stall out and die all together.

What i've fixed:
I have been having this issue for about a year now, so here is a list of everything I have fixed or changed or tested.

Fuel pump
Fuel filter
Spark plugs
Spark plug wires
Cap
Rotor
Distributor
Mass air flow sensor
Some transformer thing
We have taken the exhaust system completely off to test the cat


Solution:
I have no idea...... I have fixed everything I can think of that has to do with engine power loss. And I'm completely stumped on what I need to do because replacing the engine is out of the question.

If any of you know anything or have something you want me to try please let me know. At this point any info is good info.

Thanks,
Sar

P.S. if another thread like this one already exists please link it so I can read through it thanks.

 

[Sarhan]

Update

The update I have it that the power loss is now taking place during any heavy engine use. Some examples would be getting on the highway or pulling a trailer.

Thanks
Sar