Please help, this is becoming a parts dartboard!

Customer brought the vehicle to me having just purchased it and was experiencing it occasionally going into limp mode and storing an overboost code. The map sensor was blocked solid and replaced with a genuine Bosch part. The vehicle seemed ok for a couple of days but eventually the fault reoccurred. I have been unable to fix the fault but it seems to surge slightly where the boost is being regulated under hard acceleration. It only ever goes into over boost on very light throttle or when you have slowed to pass another vehicle in the lanes. Below is a list of work carried out so far.
Map sensor replaced
Vacuum lines checked
Boost hoses checked
Boost solenoid replaced (wiring checked)
VGT turbo checked/calibrated (had been tampered with)
EGR stripped, cleaned actuator tested
Barometric pressure sensor replaced as readings seemed slightly high