Ok!  2004 Silverado, 5.3, auto, 4wd, rusty, 300,000 mi.  Check engine light was on for engine misfires in both cylinder banks.  Mechanic said head gasket leaking coolant into cylinders. Not repairable due to cost.  Coolant WAS disappearing, so put that guaranteed or double your money back stop leak green bottle stuff in and it worked. Truck no longer consuming coolant.

So, scanned for codes and there's only one.

One misfire:  cylinder 7

Checked plug, replaced it.  Still misfire in 7

Swapped #7 coil pack with #3. Misfire moved to #3.  Aha!  Replaced coil pack with new on #3.  That should be it, right?

Misfire returned to #7.  WTH.  So... maybe the plug wire to  #7 got abused by bad coil pack.  Swapped plug wire #7 with #5.  Now the code is P0300

So the code history is P0307, P0303, P0307, P0300.

Anybody got a rationale for "The Case of the Traveling Code"?  Thanks