Cdf26501

2016 suburban LT. I am having problems with the vehicle going into limp mode. When it kicks in top speed is 35 miles an hour. It's been back to the dealer 3 times now and all 3 times they say they can't duplicate it while they're testing it. It doesn't kick any codes nor does the check engine light come on. You can being going 70 mph and it will kick into limp mode with the accelerator to the floor and it will slow down to 35 mph. I am starting to get pissed about this. Can anyone help me out with this.