Ben, the description of your car does not give us any clues how t assist you. Please give us the year, engine type, trans (Auto), etc. Your location would also help.
If the car's under warranty, then back to the dealer with a loan car until the problem is found and rectified.
What part did they replace that is not warranty covered. Whilst most service depts try to be helpful, don't take any cr*p from them, as some do tell porkies just to get you to go away. We don't tolerate that here and if caught the dealer is named and shamed, conversely, if you think you've been dealt with professionally and courteously, we also would like to know the dealer involved.