Miss or slip over 55mph

Have 2002 Mercury Villager. When I drive over 55 on the highway, wether or not the cruise is on, we feel a jolt, like the engine slows, and then it is ok for a few minutes and then it does it again. When we drive at slower speeds, it does not do it. It feels like the transmission is slipping, but I am not sure. I was told that spark plugs could do this as well?
Thanks