I've had this car behaviour since I bought it, it didn't seem a big problem.
It's a 166 2.4 JTD 20V, same engine as in the 156.
I've checked inlet hoses (good), fitted new maf, map and vnt regulation valve (just in case, all genuine parts not from eBay), checked vacuum pipes (cracked pipe on the outlet of the vacuum pump, fixed), totally blanked EGR with metal plate (not in the ECU yet) repaired swirl bar with brass kit and replaced every filter and oil in the car.
I still have this strange behaviour, flooring the pedal on 1000rpm, the car slowly builds boost, at 1300rpm it starts to pull some, at 1500 it reaches full boost (1.3 bar) then sudenly it drops to 0.8-1bar (the power suddenly goes down) and it starts doing boost again, to achieve again full boost at 1800-1900rpm, and it stays over the rev range.
The vnt actuator moves freely when plugging/unplugging the vaccum pipe, although it needs around 2 seconds to fully "suck" the actuator back into its place.
Check this pic for the detailed boost drop:
This is on a full run, check the vnt %, may it be a stuck VNT mechanism/actuator not moving freely, therefore causing overboost?
Or is it meant to be like this?