Engine shakes when operating at cold temperatures

Change the rest of the plugs and wires. At that mileage, if they have not been done yet, then it is overdue. Also, the wires can look fine to the naked eye, but the insulation could be wearing thin and allowing sparks to jump from one wire to another and from the wire to any piece of metal that is grounded to the frame (most of the metal in the car is grounded in some fashion). When the engine warms up, it does not need as strong of a spark to run well, so it runs a little better after it warms up.