05 freestar hesitating

our 05 freestar started to hesitate (sputter) when climbing a hill about a year ago but no engine light came on. now the van seems to do it at take off and sputtering at idle but when it finally comes to highway speed it runs fine. my wife (she drives it) says she puts it into neutral at stops now, otherwise it seems like its gonna stall.
the engine light came on (engine light comes and goes) and the code said a missfire on "x" cylinder so i changed the plug on this cylinder only, but a few days later same story. the problem seems like it started to happen when there's a load on the engine, i.e. climbing a hill, accelerating, but its getting worse!!
wondering what to replace next? feels like its spark related to me but why would it run ok when driving on a level highway at say 100kms hour.
the van got approx. 230000 kms on it. 4.2 liter
any suggestions would be appreciated.

When was the last time you had all the spark plugs, wires, and fuel filter changed?( A Tune Up) I would do that 1st if its been a while.

Check your maintenance records to see if your on schedule.
Doing regular maintenance can weed out and prevent problems.

1) Replace Air/fuel filters.
2) Clean MAF sensor.
3) Clean throttle body.
4) Have a back pressure test done on your catalytic converters.
5) Tune-up - plugs, and wires.

Do one step at a time until your problem is resolved.

our 05 freestar started to hesitate (sputter) when climbing a hill about a year ago but no engine light came on. now the van seems to do it at take off and sputtering at idle but when it finally comes to highway speed it runs fine. my wife (she drives it) says she puts it into neutral at stops now, otherwise it seems like its gonna stall.
the engine light came on (engine light comes and goes) and the code said a missfire on "x" cylinder so i changed the plug on this cylinder only, but a few days later same story. the problem seems like it started to happen when there's a load on the engine, i.e. climbing a hill, accelerating, but its getting worse!!
wondering what to replace next? feels like its spark related to me but why would it run ok when driving on a level highway at say 100kms hour.
the van got approx. 230000 kms on it. 4.2 liter
any suggestions would be appreciated. In addition to what has been mentionned here you should take a close look to the coil packs which is a regular issue on both Freestar and Windstar specially if this has never been replaced. By the time some cracks on the coil pack are the cause of engine misfire, which could be intermittent at the beginning but increase to really cause cylinder misfire and give a cylinder misfire code. I did replace it on both van. While the coil pack can be remove without removing the cowl vent panel, I recommand you remove it so you will be able to uscrew it from the rear valve cover ( 3 bolts) plus disconnect the wire harness, and see it. Most of the time there is one or some cracks on the top of the unit, sometime between 2 cylinders.

Agree with Catvents. Check the coil pack. I had one go bad. Mine had a slight crack on the bottom. After testing between the low an high voltage plugs I found that it was a bad coil.