Vacuum leak. That hose can also bubblegum up and suck a small hole in the side to make the idle erratic as hell. One day or week it runs fine then just goes to crap out of the blue. Focus zetecs are known for it, at least the earlier ones. The PCV entry at the very bottom back of intake. I chased mine for a year before stumbled across it. The motor would run so bad at times that it would shake visibly as if it were going to jump right out of the mounts. No CEL ever registered, but finally did a realtime O2 sensor scan that showed no switching, stuck low volts at idle fuel trim limit. In short, most likely vacuum leak.
How were spark plug wires 'checked'? No more than 5K ohms per foot. Just changed out one on my '02, it had infinite resistance. Still worked 99% of the time, would just miss occasionally while calling for max power up say in passing situation. I've seen bad plug wire make OD disappear on a Contour zetec, that was wonky. It didn't even seem to run bad but changing the wires fixed it. These wires break the conductor internally real easy when pulling them for maintenance like plugs even if you're gentle.