Duramax dies going down highway

While I am at it it will ask another question. My brother has a 2006 (?) Chevy 3/4 ton or 1 ton, cant rememer, with a duramax in. With somewhere around 300000 miles on, thats not a typo, he does a lot of driving. Sometimes it just dies going down the highway. Up hill , down hill, on the flats it dont care. There is no check engine light. sometimes it will start back up and it will be ok and sometimes he has to start multible times. He has had it to several shops with no luck.
I wish I knew more details but just wondering if anyone has heard of this.

My dad had a similar problem with his ford. It turned out that the injector harness that is under the valve cover was damaged. The truck would just die at random or get partial power. The only other thing would be to check the fuel pressure and pressure regulator.