Over boost while jaking

[Northern guy]

I have a Isx 2250 that is a drop in 4500hrs ago warranty is done was de mandated a year a go and has run flawlessly. Happy with the results with it. A few months ago I developed issues with the vgt actuator it had defeated and coded out just with the service indicator not stop engine. I took it into my local Cummins shop for repair got it fixed. Was gone about a hour and brought it back with a stop engine light. What’s looked at again and decided that the vgt actuator was faulty. Changed it and went back to work. A few hours later I was back again with codes on the turbo and now the engine is over boosting while jaking and not performing as it should on the jakes settings it over boosts while on the third stage up to48 psi at times, then will blow the boost back out the air cleaners when you touch the throttle. Back at the shop they recalibrated the vgt actuator and inspected the turbo. The vains move freely. Back to work. Well back into the shop as of stop engine light again. More diagnostic on engine changed all the sensors that may have or have not needed to be changed and problem still remains but with no engine codes. So the shop has a test turbo that they tried and didn’t solve the problem. So replaced it with my turbo. Then took off the new actuator and put my old one back on and doing the same as the new one. Now with out any codes as when I brought it in. Now they try a test ecm that they have with the same programming
still reacting the same way over boosting while jaking. Now afterwards my ecm is back on and it is still the same. This is all done over a space of 3 weeks as they are out of ideas. And the the say they are out of ideas on what to do. Basically come and get your truck out of out yard.
Seams to me one the jaking issue to me is it doesn’t like high rpm jaking works better when starting at a lower rpm and ramping it up. Seams like when jaking it Down to mid range rpm it wants to vibrate a little. I’m thinking that it’s the vgt not knowing where the chains are supposed be set at and are fluttering. The engine runs flawlessly otherwise. Boost is where it’s supposed to be while pulling. Any ideas out there ? Out of patience and don’t want to run it till a major issue shows up but may have to.