2008 Nissan Frontier 4.0 Hesitation

I have a 2008 Nissan Frontier 4.0, and on February 14th 2011, my wife was driving up a hill on the freeway and it started stalling (kind of like wanting to shut off) so she pressed the accelerator and made it home fine. The next morning, I started to test drive it to see if the problem continues, and sure enough, it did the same to me. I drive about 25 feet and it started hesitating and shut off on me. Since the vehicle is under warranty, I called the warranty company and had them tow the vehicle to the dealer for service. The dealer "Said they check the vehicle with their equipment and its running flawlessly". What can the problem possibly be? Im starting to think that it was a simple little part and that the dealer replaced it and now they are telling me that nothing is wrong with the vehicle so that I can pay the tow fees and rental car fees. What do you think? How can I verify this situation?