Maxxforce 13 high turbo boost low power

The boost is one of the things Navistar made overly complicated for no reason. To control the boost, there is an "Air control valve" mounted near the turbos with a bunch of lines connected to it. It is supplied by truck air pressure. When the engine senses boost is getting high, it will command that valve to send truck air down to the waste gate to open it and dump boost out the exhaust. Sounds like it is working intermittently for you. The engine will derate when boost is too high, so I am guessing that is low power. You can do a basic test yourself. At key off, the waste gate is fully closed. You can look at it and see the little rod at the back of the air pot. With full truck air pressure charged, key on, and the valve will energize that pot with air and push the rod out about 12 mm. Make sure it is moving. Sometimes the little air pots will rot and leak, or air line leak or linkage itself will seize up and stick to prevent it from opening.