Accessport uses the stock ECU fuctions, and as such, can adjust to various air temperatures.

Sounds to me like its hitting some sort of fuel cut. I wouldn't expect the misfire to do it, but its possible that when the ECU goes open loop it puts just enough fuel in to make the plug misfire. Or it could be a combination of the boost levels and the plug that's making the ECU think its knocking hardcore.

My suggestion: Put in the new spark plug, and reset the ECU by disconnecting the battery, not through the Accessport, and see if that helps (Resetting through accessport just resets CELs, not ECU learning)