Welcome, I am Chris (aka-Moose)
The first 2 things which comes to mind are a soot filled stuck EGR valve or there are coking deposits built up on the turbo vanes and it can not adjust correct to control the boost pressure. Any warning lights on, codes stored? Any smoke concerns when this happens?
----------Once you get an answer----------
1) Please realize I base my info requests and answers on what you type.
2) The more you can tell me the faster and better I can help.
3) I am not an employee for Just Answer, they pay me nothing to help you.
4) I am paid by the customer from your deposit, but only when you rate my answer.
5) You will notice it says rate to finish, rating does not close the post, or end our conversation.
6) I am going to stay in contact with you until you respond and tell me your happy or pleased with the answer. So please reply and let me know the progress on the issue and our discussion or if more help is needed.