I have a '95 GT with almost 200k miles. It's been a great car but I'm having a problem I can't diagnose. No matter what the engine speed, the car shudders and stumbles. Replaced the fuel filter recently just because it was due. Thought the problem seemed ignition related, so I replaced the plugs, wires, coil, distributor cap and rotor, and OLD battery. Before replacing the coil, the tach was reacting slowly, but unfortunately that's all that was fixed. Power has dropped significantly, and mileage isn't as good as before. What am I missing?