Shifting problems on Terrain with I4

Tonight I started driving my terrain, slowly speeding up to 55 mph. When I reached about 50 mph and the transmission was either going to shift into 6th gear or go into lockup the transmission started shifting violently back and forth between the two gears. It did this for about a second or two until I let off the gas, then was fine. My wife told me it did the same thing to her also. I only have 1050 miles on it. Has anyone else experienced this? It's going to the dealer tomorrow but I know they'll want to reproduce the problem (which I can't). I would like to have some other information with me if possible. Any feedback is appreciated.