2003 Ford F-150 Slight Bucking @ 40-55 mph

Just bought a 2003 F150. Haven't even made the first payment on it yet. When we bought the truck, the salesman told my husband that he had replaced the coil pack in the truck after buying it at an auto auction. After about a week of driving, my truck started "bucking" while in the 40 - 55 mph / 1100 - 1300 rpm range. I've been reading up on this problem and it seems it's at least somewhat common. I've read lots of different opinions as to what this problem needs in order to be fixed, but wanted to ask someone myself. Thanks in advance for your help!