I have a 94 GT with about 154,000 miles on it. When I'm at a stop, the car idles pretty rough to the point that I can feel the car shaking. When I look at the tach I can see the needle surging up and down between 600-800rpm. The idle air control motor was replaced less then 10k miles ago with an oem motorcraft part. The check engine light never comes on. Should I start by doing a tune up, replace plugs, wires, cap, rotor, and fuel filter? Any other suggestions what could cause this?