Automatic Transmission Torque Converter problems of the 2013 GMC Sierra

Initially I felt the problem as a slight "missing" in the engine under very light acceleration between 40 mph and 55mph and more pronounced under constant throttle going up grades on the highway at a speed of ~55 mph (1400-1500 rpm). The vehicle seemed to be "lugging", and then the rpm would increase by ~20% immediately for a short distance and then drop back (all at constant throttle) to 1400-1500 rpm. When driving on the highway in cruise control at 60 mph and going up a grade, the vehicle would show signs of "straining" and immediately increase in the rpm by about 20% with an attendant acceleration. Then the rpm would increase instantly by about 1000 rpm and the vehicle accelerates rapidly until I engage the brakes. My dealer observed the same characteristics and determined that it was a torque converter problem and on 07/25/2014 they installed a new torque converter, which proved to have no effect on the problem. I was informed by the service manager that GMC technical services informed him of a service bulletin (#pip4681d: random bump chuggle or fishbite sensation while cruising at approximately 1,300 rpm- October 31, 2013) that addressed the problem, and furthermore no further corrective action could be taken. On August 6,,2014 I was driving at constant speed of 60 mph on a major highway in "cruise control" and moderate traffic. While going up a long grade following another vehicle at a safe speed, my vehicle immediately jumped ~400 rpm and then abruptly changed to a lower gear and accelerated suddenly putting me at risk of colliding with the vehicle ahead of me. I quickly applied brakes and was able to avoid an accident. I immediately contacted GMC customer services who opened a case number (71-1344392355) and assigned a rep. To my case. I have subsequently been informed of the GMC company line--"the vehicle is operating as designed", "live with it"!!!.