2004 Chevy Silverado 1500 4.3

Could be leaning out under load, have the codes pulled and see what they say, as you had a flashing light it should have stored it in memory.

I already replace the distributor cap

Mi Chevy 2004 vivra y pierde fuerza después de manejarla por unos 20 minutos y flashea la luz de chek engine

First off an 04 doesn't have a distributor cap, it has coil packs. So there's no way you could have replaced it. You might want To find out where it's misfiring and replace the coils on that cylinder

This was 5 months ago, so no news is good news.

Have anyone tried changing the sensor on the gas peddle? Seems to be another bad part of gmc

Billy as a matter of fact yes they did use a distributor on some wt styles i happen to own a 2004 gmc sierra 1500 wt with a 4.3L v6 distributor style