Car won't go ovr 50mph on 2008 Hyundai Elantra

Mom's car seems to drive fine unless she is driving in the mountains. She says it won't go over 50mph but Pep Boys can't find anything wrong with it. She says fuel injectors were cleaned 2 years ago, or so. We are getting ready to move Mom and need to drive her car from CA to Indiana. Looking for solutions to what could be the issue.

Asked by for the 2008 Hyundai Elantra