Q&A

 Master

You may be having internal transmission problems, first I would get it checked to see if it has the latest software in the transmission control module. There are software updates that often address some of the shifting issues that this model has.

 Rookie

Try these two things. First, make sure you have a complete transmission service performed. In other words, the filter is changed and the fluid is replaced in the torque converter also. This is a full transmission service and requires dropping the transmission pan to gain access to those two items. Simply using a transmission flushing machine does not change the fluid in the torque converter and leaves a dirty filter. Too many times people complain of their transmission's slipping after a transmission fluid change. However, if the filter and fluid inside of torque converter are not changed, you may experience slipping. In fact, you have gained little and only put new fluid in with old fluid and are still using a dirty filter. This should be performed on a regular basis between 25K and 30K miles.
Of course have a software update done as "bretb" suggested.
Much of the time these things will correct this type of problem and are the most economical way to avoid problems later. If none of this works, then you may have a more serious transmison problem internally.

transmition filter kit

Since your car is newer with less miles try checking tranny fluid level. After stopping from a trip and tranny is hot, step on brake and shift through all gears a few times. Check your tranny fluid level and it should be in HOT zone. Add fluid when needed and repeat the next day until fluid level stablizes. My car had the same problem and got fixed using this method. Good luck!