OK. I picked up a 1997 Chevy truck 5 speed with a V6 vortec for my nephew. It has an engine code P0300 (Random/Multiple Cylinder Misfire Detected).
Starts instantly. Drives fine. Never stalls.
There is a definite misfire while running over 2000 RPM with very light load. If you create load by riding the brake, you can make it stop missing. If you take off hard, you can wind it up and go, and it runs smooth. You can drive it all day. If you shift at 1500 RPM and drive it like a grandma, it never skips a beat.
A basic tune up was done. New plugs, wires, cap & rotor. New gas tank and fuel filter. (The tank had a rust hole). A missing anti-lock brake sensor on the front wheel was replaced. No change.
I have not checked fuel pressure or much else at this point. The miss is definitely load related. No pinging. You can lug it in fifth gear and it won't ping. Once you pass 1800 - 2000 RPM under very light load, it skips.