This transporter originally came to me after having a foreign object dropped into the inlet manifold causing damage to cylinder 1. I have rebuilt the engine (piston, valves, guides etc) and the turbo was sent for inspection and consequently repair due to impact damage.
Since covering around 2,000 miles it has started suffering from underboost P0299. I have done all the obvious checks, vacuumed everything, smoked it, pressure tested intercooler and hoses and the boost control solenoid was replaced at time of rebuild due to being noisy. I have also tested egr and even tried blanking it but made no difference. when reading live data the actual boost is lower than the desired boost yet vacuum is being bled off from the actuator via the boost control solenoid. I have tried it up the road with a vacuum gauge T'd into the actuator and I start off with approx 15hg of vacuum before it slowly decreases and I can cause an overboost condition by manually increasing vacuum.
Any help with fixing this will be greatly appreciated.

The second image live is live data where it went straight into limp mode under acceleration 6th gear approx 2,000 rpm.