It could be a catalytic converter, fuel, or air intake issue. To test the cat. start the truck and let it reach normal operating temp. run in park or nutral at 2500 rpm for 3 min. after 3 min continue to run at 2500 rpm and measure the temprature at the front of the cat and at the rear of the cat. It should be at least 200 degrees hotter after the cat. If not replace it.
check fuel pressure and volume to make sure you are within specs.
Also on the back of the distributor is a ignition controll module that go out all the time. The have a simular reaction. If you have not replaced it in the last 50000 miles it is time to do so.
If you have anymore questions feel free to email me at
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