Shaky Situation.. Please Help!

I have an '03 Cavalier and haven't had any problems until now. I had no indications (that I was aware of anyway) of trouble until yesterday. My car shakes horribly, especially when it's idling. When it down shifts, it shifts out really hard. It feels as if it's on the brink of dying every second that I drive it. We had a diagnostics thing done and it says that it's randomly misfiring on multiple cylinders. My husband replaced the spark plugs and the coil and neither have helped. Does anyone know what could be causing this and how to fix it? Thanks so much...