Seems to be a common issue with 6+ speed automatics, having a hard time deciding what gear to be in. I see it in trucks too. Some have of these issues have been addressed with computer software updates, but i am also going to suggest getting a tranny service done (flush) as well. And having it serviced every 20,000-30,000 miles after that going forward. The idiots at GM and other manufacturers are sometimes claiming that it is "lifetime" fluid and doesn't need servicing. They even remove the dip sticks.. those dip sticks..