I have a similar phenomenon here where I live in Eastern PA (Tamaqua) to be specific. A solid line of storms can roll across the entire state, as well as all the Appalachian ridgetops, fully intact, until it hits I-81 just to my west. At that point the line will weaken and fall apart and then just to my east will reform and slam the Allentown/Bethlehem area and points beyond. It happens about 90% of the time.