2003 Sorento ex with no acceleration at times

[threespeed]

My son's sorento has started having a problem with acceleration. His girlfriend was driving it last night and when she stopped at a stop sign the truck would not accelerate. She was pushing the gas pedel but the engine was not accelerating. She turned the truck off and re started it and was able to get to her sisters house before it did it again. I went over there to check it out and it ran great. What could be the problem. The check engine light was not on. Any ideas?