2004 Chevy Silverado 1500 4.3

A flashing check engine light is warning you that the engine is misfiring and damaging the catalytic converter. Sounds like it's misfiring under load. This is an ignition system problem. Check your ignition coil since a tune up didn't seem to solve the problem. Did you replace the distributor rotor? You didn't mention that. If not, replace the distributor rotor and check that ignition coil sounds like it's failing. HTH. -Jim

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