1992 ford conversion van shifting hard

My son took the van to a transmission shop and the mechanic took it for a ride after explaining all the usual problems with sensors causing hard shifting. His conclusion was that it was a misfire in the engine. I would not have suspected that since plugs, wires, cap & rotor were just changed. But he suggested changing the coil. His thinking was high rpm when you get on it it shifts well but at lower rpm the coil was not putting out sufficient voltage witch caused a slight misfiring and a hunting of the trans before finally shifting hard into gear. Will check the coil out tomorrow.