Transmission Gear Slipping problems of the 2013 GMC Sierra

Hello, the date and milage above is not actual date of first incident, but is the first time I took it in for the first oil change and vehicle inspection which also was the first time I complained to the dealership about the issue. My transmission slips between 2nd and 3rd gear at least 4 times a month. The dealer told me that if I cant prove it or if it doesnt do it when they drive it then they cant perform a diagnostic test to fix the problem. The seller who sold me the vehicle called me twice to see how im enjoying my truck and I complained to him both times about the issue and he told me the same thing about my issue. I even left voicemails to him about it and he never returned my calls until I made a year (3/16/14) owning my truck and claims he dont remember me telling him this the first time. Im a strong gm fan but I feel im not being helped with my issue and its annoying that I have to drive my truck cautious guessing will it slip this time or will it slip next time? if its professional grade then why am I having this problem? I use my truck for family vacations and I know anyone with the same problem would not drive this truck far away from home with this issue. It does it when it wants to but its just funny how im trying to figure out when it will do it again. I get all these emails asking me to rate the dealership 5 stars and to rate the truck but I cant even get my truck checked out. Then they tell me they wont give me a rental which I would need another truck as a rental. This is sad I spend 30 grand on a truck I cant get worked on unless I have proof.