New 3500 express smoking Black/white

I just purchased a 2006 express Van With the Duramax deisel. With just over 1000 miles on it pulling a 6X12 open trailer with a golf cart on it, it started blowing white smoke when I reached slightly over 70 mph. If I kept it below 70 no smoke. 2 days later with no trailer after 60 miles on the interstate, When I got off the interstate it seemed to be running rough.I gave it some fuel and then a big cloud of black smoke. It is now at the dealership for the second time now.The first time they said they reprogrammed the computer and that would stop the smoke.Well that didn't help. They can't figure out whats wrong with it. Now if smokes periodically. Sometimes it runs great sometimes it smokes. Any ideas. Thanks.

Could be the EGR being over active, or I guess it could be the turbo vane positionin sensor. It's not a super common deal, but the turbo position sensors have failed for some. See if the dealer will do the turbo position learn for you.