p0299 under boost and rev fluctuations

Hello
I have an Audi A3 8P1 170HP with an engine code BMN. I know its not a skoda but some skoda have the same engine in.
The issue I am having is with the boost of the car which is appalling and revs fluctuation.
The car boosts but not in a consistent way. When the turbo kicks in the car will boost all the way up to 3500rpm-4000rpm max then cuts out, the engine is then capped to 3000rpm where the revs start fluctuating between 2800rpm-3200rpm. When this fluctuation happens you feel the car acelerate harshly and then decelerate harshly constantly which results in a jerky ride. When the car does boost all the way up to 4000rpm, the same harsh accelerations and decelerations are felt constantly, as at times the revs will fluctuate when accelerating. If you drive the car long enough less than a mile you eventually get the coil light flashing, where the engine will be limited to 3000rpm and will not go above it until car is restarted where the cycle repeats. At times when the engine is capped to 3000rpm alot of thin black smoke is released from the exhaust pipe
You do not have to drive the car to see revs fluctuate, when idling if I put my foot to the floor on accelerator. The revs will go all the way into the redline 5000+rpm constantly, but after a short while about 5-10 seconds, the revs gradually decrease to 3000rpm and then the revs fluctuate between 2800rpm-3200rpm occur, even though accelerator is fully depressed. Eventually the coil light will start flashing followed by thin black smoke coming out of exhaust pipe if I keep the accelerator depressed. Revs will be capped until I restart the engine where the process then repeats.
When checking for codes on VCDS. The Ecu throws a code for both situations are the same if I reset the codes and try revving on idling or driving the symptom repeat again.
VCDS code:
000665 - Boost Pressure Regulation
P0299 - 006 - Control Range Not Reached
Freeze Frame:
Fault Status: 11000001
Fault Priority: 5
Fault Frequency: 2
Reset counter: 40
Mileage: 123627 km
Time Indication: 0
Date: 2019.06.14
Time: 00:31:03

Freeze Frame:
RPM: 3040 /min
Torque: 296.0 Nm
Speed: 45.0 km/h
99.45 %
Voltage: 14.01 V
Specified Pressure: 2492.1 mbar
Actual Pressure: 1704.0 mbar

Based on the code I have tried the following.
I have checked for boost leaks and found none all turbo pipes are clean no holes no damages. I checked all vacuum lines to the turbo and other components which are fine no split no kinks or damages.
The next step I did was replaced the map sensor and the maf sensor from a donor car no change in symptoms but donor car still runs fine mine has not rectified.
I have put in a new n75 valve a new crankshaft sensor and a replaced the camshaft sensor from a donor car still problem persists.
I have checked all the wiring from ECU to the following sensor connectors:
The electronic turbo actuator sensor, N75 solenoid valve, Egr cooler solenoid valve ,swirl flap solenoid valve, crankshaft sensor, camshaft sensor, DPF pressure sensor, egr valve, throttle body valve (bit in front of egr valve),accelerator postiton sender 1 and 2, all of the injectors and glow plugs. All the wiring is spot on with no shorts and continuity of each wire is solid.

After trying all of this I then took the car in to Audi where they ran a diagnostic and the same code showed
P0299.turbos/super charger underboost
picture bellow is audi scan from main dealer
audi a3 8p1 full scan 05022020.txt

Audi has checked the vacuum lines and have said they are fine and have done a boost leak test
Where no leaks were detected, and have said the turbo is fine. they wanted to further investigate and spend another 6-7 hours at rate of £105ph which is too much to spend atm.

To further confirm the turbo is fine. I removed the turbo charger and took it to a turbo rebuild shop
Where they disassembled the turbo to check internal condition and they said nothing is wrong internally and all the vanes are clean with no carbon or soot build up. They also checked if the turbo is calibrated correctly and the electronic sensor and actuator are all working all which has passed and they provided a certificate for showing what flow rate it is producing.
Turbo is refitted and the problem of the is still there.

Could anyone out there please help me as I am hitting a brick wall and do not know what else to do with it.

cheers in advance