could just be overboost, but this usally happens when its been set up on a warm day then you go out on a cold damp morning and "bang" "pop" overboost,

you say stg1, boost should be no more than 1 bar peak, dropping back to just under 1 bar.

other thing to check of course is plug gaps, as they do open up with age and with hotter ambient temp they could be breaking down when she comes on boost. i'd whip them out and check the gaps