Same exact thing happened to me in my 2010 Silverado at about 60k, it freaked me out too. I did some research and it turned out to be a bad throttle body. I ordered one off amazon($140) and installed it myself, never happened again. About a year after I did it GM put out a recall on the throttle body, I`m in the process now of getting reimbursed for it. Check into the recall, it explains your exact situation. Honestly it took me about 15min. to swap the throttle bodies.