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I am sure you don't want to hear this, but before you throw money at parts that are not absolutely obvious, you should invest in the Autoenginuity scan tool and ford expansion package. It's not cheap, but will save you money if not on the first repair, at least on the second one.
What you described could be a couple of problems. it could be a sticky EGR valve. You can clean that and give it a try for a couple of dollars worth of carb or brake cleaner.
The boost pressure isn't necessarily wrong. The computer calculates the amount of air molecules going into the engine and regulates pressure based on that. If it's cold, you may see lower boost pressure than when it is hot outside. That is because 27 PSI of boosted very cold air produces a lot more combustion pressure than 27 PSI of hot air. Ultimately it is regulating the amount of HP being produced to the designed capability.
It could also mean that it is low pressure which could be cause by a short in the wiring to the VGT solenoid. You would probably get a boost control code if that happend though. The short can be hard to find. I had one that was about 8 inches down in the protected corregated wiring cover down beneath the turbo level.
It could also be indicative of a sticking VGT. The veins and mechanism can get rusty and carboned up causing the veins not to move correctly. Ours would do what you described only worse. you could floor it and get good boost, then when you let off, the veins stuck wide open. this leads to almost no turbo response until you have it floored for like 15 seconds and eventually the turbo spools up and starts producing boost again. ours would eventually unstick and work again. Cleaning turbo was really easy and it works like a champ now.
Back pressure could be off as well. Could be a stopped up line that leads to the sensor, or a bad sensor. All of these can be diagnosed with AE or comparable scan tool.
Let us know when you find the problem.
I don't see one, but I wish admin would put a "solved" section in the forum that would allow us to move the thread so that people can see how a problem was solved. Too many of these discuss for a long time and then there is no resolution to the problem.