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[RatherBNtheWoods]

Mine started a similar type thing last summer. It would be fine for a while then go to under-boosting and over-boosting and puff/fart from under the hood. Ford warranty paid to rebuild the turbo and clean up the exhaust side all nice and shinny, the VGT (what ever moves the vanes) the EBP and the little tube it mounts on.

Next trip same thing, they put a MAP/BARO sensor on it.
Next trip(out of warranty) same thing. When it would "act up" I would have to drive via the boost gauge. Get too high on the boost gauge and the engine sounded like it was really straining,( like it had a potatoe stuck in the tailpipe) until I saw the boost fall down(as if something came unstuck) then it would be fine until the next time I needed near max boost.

So, short story long, I'm at a rest area with the hood up cursing the darn thing, (you don't make good time towing the 5er without going over 15-20lb boost) when I noticed the shop had routed the vac line to the heater core valve around the MAP sensor wrong. So, being anal as I am about hose routing and everything in general, I routed it correctly. When my fingers/hands were near the hose that went to the MAP it moved quite easily on the MAP sensor. I thought to myself, you know. A hose that holds upwards of 30 PSI should be tighter than that. And,,,I pondered, if it were loose enough to leak, the PCM would be calling for more boost,,,and hum...I cut the end of hose off just a little, to get to new unused part of the hose, and plugged it on the MAP..........And......
Not another fart/poof/puff or overboost/underboost all the way home!
Since then i replaced the hose, but still haven't had another episode yet.

In short,,,check the little 3/8 or maybe 5/16 hose that goes from the manifold to the MAP sensor. Needs to be good-n-tight. Worth checking anyway.