I have a 2001 Chevy Silverado 1500 v6. It has 163000 miles and I did a tuneup and did a fuel pump replacement. Also replaced all o2 sensors. Crankshaft posision sensor I replaced myself on all of this and checked to ensure they were on secure. I still have a p300 and it only comes on on a harder ecceleration.
The 4.3 still had the single ignition coil with the ignition control module mounted on it.
You didn't say how much of a tune up you did. I'm thinking probably spark plugs, spark plug wires, cap, and rotor.
From your description it sounds like the misfires only happen when you load up the ignition components.
On yours I'd be thinking secondary ignition stuff. Ignition coil, and the things I listed above.
The other thing is the ignition control module. Often the first sign of it failing is misfires under ignition load
Well, I did the ecm with the ignition coil. They are attached. I did wires, plugs, coil with ecm, crankshaft posision sensor, and o2 sensors. 🙁