Consider too a vacuum leak showing up at the much higher vacuum of decel ONLY.
I mention it because the zetec has a crap super thin gasket section at the idle bypass port at the joint between TB and intake.
I've had that thin section pull out sideways to then reseat back in place before and it gave me fits finding it. The entire TB does not sit right on the intake surface flat, rather the bolt holes in the corners have slight 'stands' molded into them to hold up the TB assembly roughly .010" or so high on the intake. That airspace is what allows the gasket to pull out of place, it is not hard pinched to not move at all. The gasket as well with age loses a whopping amount of the physical 'body' stiffness the material has normally that would stop that, it becomes extremely flexible. I could see by the coloration of the silicone there that the gasket right there had been sucked out in the wind at least part of the time, it was twisting sideways out of place. Kooky, man.
My problems there were not the same as here but with a little imagination I could see the gasket moving at decel only to leak to help kill engine then falling back into normal place to run right. Far-fetched but not impossible.
Fix the driver first, then fixing the car becomes so much easier..........