Transmission/ignition problem?

I have a 1994 Ford Taurus GL 4 dr sedan 3.0L with auto transmission that has 110K miles. The problem is that when I'm driving at a constant speed (giving the engine just enough gas to maintain a steady speed, but neither accelerating nor decelerating) the car seems to "stutter/shutter". However other than that the car runs fine, engine idle is smooth, acceleration is fine and the gas mileage is also good, (or as good as can be expected).

At first I thought it was an ignition problem so I replaced the wires, plugs, cap & rotor. It seemed a little better but wasn't fixed. Given that the engine seems to run well otherwise I'm wondering if it might be a transmission problem as if the transmission is seeking the right gear while driving at a constant speed. It seems worse at constant speeds between 35 - 45 mph, but will also do it at constant speeds right up to 65 mph, however over 70 mph I don't seem to notice the problem.

The problem has been going on for a while and although it doesn't seem to be getting worse, it also is not getting any better.