What tool are you using to read the codes? Is that tool capable of reading cylinder-specific misfire codes? Or does it lie about cylinder-specific misfires by telling you they're "random" misfires?
The switch from open-loop to closed-loop operation shouldn't take ten or fifteen minutes.
Wild Guess: an electrical or electronic component is failing when it's fully warmed-up. I'd be testing the ignition coil for spark using a spark-tester calibrated for HEI, when the truck is hot enough to act up.
You may have a failed ignition coil, or ignition module that only causes problems when it's been run long enough to fully warm up. Similarly, the ECM could be screwy when fully warm, or you may have injector(s) that cause problems when the solenoid coils get hot.