2003 Hesitation Problems - Please Help!!!

Whenever I reach 55 to 60 mph, the car's rpms drop and the car hesitates. The car has 108,000 mile, so I thought that the plugs needed to be replaced. I did so and the problem is still there. I also had AAMCO check out my transmission and they found nothing wrong on the their "trans scan". Their is no check engine light on and I have no clue what could be wrong. Can someone please help?!!!!!!!