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99 silverado 4.3 hesitates under load/no. 5 cylinder misfires

I replaced the spark plug wires, the coil wire is about 3 years old and the spark plugs have about 3000 miles on them. The truck has 172,000 miles on it. Starts right up and idles fine and no problem reeving it out of gear. A soon as I put it in gear it bucks. If I let off the gas a little, it smooths out until I step on the gas. All gears and at 70 mph I can not use 5th gear. It is a five speed.Dealer ran codes. Number 5 cylinder misfiring and they thought it was the plug wires but had none in stock so I replaced them.

Have you inspected/replaced the distributor cap? Chevy had a TSB (technical service bulletin) that covered a few years of 4.3 Silverados that addressed an issue with the A/C line that passes directly over the distributor. Over time, condensation dripping from the line would cause corrosion on some of the cap terminals. The “fix” was to relocate (bend outta the way) the offending line and replace the distributor cap/rotor.
You won’t get any love at the dealer this late in the game given the age/mileage so you’re going to have to fix this one on your dime, but it’s not the end of the world.
I’d replace the cap/rotor, check that #5 plug wire for resistance (just because it’s new doesn’t mean it’s good), and replace the #5 plug for good measure.

cleaned injectors and replaced rotor and cap. Runs like new.