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1994 Grand Marquis LS shaking

My old mercury has started shaking sometimes when I am driving and will continue when I am stopped at a red light. It feels like it is going to cut off. Can someone please tell me why it is doing this.

What mileage, and when was the last time you changed the sparkplugs, and the sparkplug wires?

Thank you for your response. It has about 92,000 mileage and the sparkplugs were changed years ago, I do not remember exactly when. A reply will be greatly appreciated.

I assume that the spark plug wires were never changed?
I also assume that the check engine light is not on?
If the light is not on, then the plug wires, plugs, coilpacks, and fuel injectors would all be suspect. If the light is on, then many more things are possible too.
It sounds like you haven’t done any troubleshooting on this.
You can throw parts at it, or you have do the usual checks such as a compression test and scoping the ignition and so on…
Also, for problems at idle, the Idle Air Bypass Valve might be the problem.

The spark plug wires were changed, but it has been a long time. The check engine light has been on at times. When I drove the car this Tuesday (January 6, 2010) it did not come on. Thank you so much for you help.