Multiple misfire codes on GM has been a growing problem I've seen on OBDII cars. The first few were nightmares but lately I've taken a different approach. I set my trace scope to 14V over 1 second scale. I set the probes on the crank sensor. At idle I've seen a solid pattern, when I snapped the throttle I noticed the wave voltage drop instead of just tighten up. After replacing the sensor I repeated the test and it looked normal. A tight pattern holding 14v under snap throttle conditions. So if you get erratic misfire codes and you have a trace scope, you may want to check the crank sensor pattern for a tight pattern with no voltage loss as you increase the RPM.