Nissan Patrol 4x4 (Africa)

My ZD30 engine had a turbo failure recently and the turbo (Garret) was reconditioned. I went on a holiday trip and for about 1000 km the engine was running smoothly. One morning when I was cruising at slow speed through the Ithala game reserve the car suddenly lost power, boost went to zero, there was a muffled bang like the misfire of a petrol engine, the exhaust puffed out a cloud of smoke and after a few second the engine ran normal again. After another 400 km the same problem reoccurred but this time the lack of boost did not go away. I checked the EGT and found out that the car builds up boost to about 0.4 bar, after that the boost drops to zero. Had the car towed to Joburg, the guys who changed the turbo took same out, had it checked and said all is good after they did a testdrive. When I wanted to pick up the car, again no boost. Not too sure if the guys know what they are doing. Could this be a waste gate failure? Any suggestions how to proceed from here?
Thanks Hannes

Check all the vacuum hoses

I had some variable boost on my 2014 3.0CRD - turned out to be the vacuum hose that connects to the actuator on the turbo - it was soft at the end : as soon as the turbo got hot, the hose end expanded and lost vacuum. I cut 15mm off the hose end and refitted - BIG problem solved with my Leatherman Wave....

Thanks Dungbeetle,

the vacuum hoses are all new. Will keep on looking...