On that year Duramax, it's normal to get up to 230 degrees water temp, fan kicks on then cools down and starts over again. As others have noted, kick the tranny in manual mode to be around 2,500 RPMs up the hill and it will run cooler. If it's truly overheating, you'll get a fuel filter error message. I had that once when I didn't shift down early enough on a long 35 mph pull.
To reduce temps, get a cold sir intake. I had an S&B and it ran cooler after getting it. You can also get bigger exhaust to get the air out quicker.
