if you are still on warranty, bring it to your local dealership so they can run a diagnostic on it... they might also want to run a road test to see if they could duplicate the problem
before they do anything to your car, they should explain to you that if they find something wrong with your trany, and they can relate it to any of the modifications you currently have on your car, that you will be responsible for the $bill$. But if you haven't done modifications and they find it being defective, then they will cover the repair costs.
best of luck