I purchased my GMC sierra new in 2009. Its one with the v-6. Well over the last month ive have started to notice its starting to shift from first to 2nd roughly. It even does it from 2nd to 3rd also. Its an automatic. Its got 308,000 miles on it. Been very happy with truck. Its doesnt do it all the time. I work like 8 miles from my home and it wont do it all when I just go back and forth to work for a week at a time. I notice it does it when 1. when I drive it over 45 minutes or 2. when I am pulling my trailer. Also when he does it I can stop turn it off start it back up and drive and it will magically go away. Any i deas whats it could be? Hopefully its not something to major. Thanks in advance!