Shadow, a couple of things, although they are SVX based, I would think they would be similar.
I believe your GTP is equipped with a Mass Airflow Sensor (as are most cars these days). It could be the culprit and just be "acting up" intermittently. Also, you may try removing your negative terminal on the battery overnight to re-set the ECU (not sure this works with Pontiacs, but, it can't hoit
) That may take care of any "learning" the ECU has done under higher octane performance. Again, these are just guesses.