I changed the plugs on my 2003 Windstar but it is still missing. In Neutral, it has a rough idle sometimes. When I put it in Drive, it starts to buck a little more. The farther I push the gas pedal, the better it starts to work & doesn't seem to miss as much. What could be the problem? I've been told that it may be the gas filter.

 

My first suspicion would be a possible vacuum leak. Check all the vacuum hoses and connections. Be especially careful when checking the PCV system hoses. You could have one that has either cracked or become spongy with a leak.

Many times a miss doesn't seem as bad when the RPM is raised, but it is still there.
There could also be other problems.

 

When you change the plugs and assuming you did the wires as well, you should also think about changing the ignition coil. That's what I do anyways. good luck

 

Of course the filter needs to be checked. At the same time, you should also check the spark plugs and injectors for proper functioning. A pressure test may give you an idea about the actual working of the engine. But first, change the air and fuel filters. This will help you surely.