Turbo Diesel Going into Limp Mode

I have a 2012 VW Passat TDI and I have an issue where it will go into limp mode if I hit a certain RPM. It ran great before it did this. I feel like it could be not getting the airflow it needs. It still runs normally before it hits limp mode. I am hoping there is somewhere I can get it fixed without paying an arm and a leg.