Assuming you are coming to a complete stop after backing up before putting the car in a foward gear, it sounds like the transmission has not fully engaged into gear. This may be caused by "low line pressure", this is the pressure of the transmission fluid that actuates gear engagement. It can be adjusted by your dealer through the ECU. There are other causes for this late gear engagement but this is the most common. I would have your dealer check it out, a little bit of time spent to check it out and fix the problem now can save big $$$ later.
