I doubt it is the warm up regulator because the car was already at operating temperature or above.

It is C.I.S. - not electronic.

You may have something on the backfire theory though.
It was extremely warm that day - around 105 deg. F and the car had literally just been used around town so it was hotter than usual as well. I had filled up the day prior to this...... and I always use premium per instructions.