How to repair a check engine light P0304?

99 Mercury Cougar 2.5L V6 100K+ helping friend out, cylinder 4 missfire, replace spark plugs and wires found a few broken hard plastic vacuum hoses fix those too. Clear code idle great, test drove runs smooth that was the 1st trip. On 2nd trip light came on but flashing 5-6 times random missfire and stayed off, scan for codes again, none to be found but still runs great no hesitations or jerks nothing, rechecked wires their plug in correctly. Scan tool is picking up the missfire every 5-10minutes or so. But the light wont stay on just flashing. Even without the light flashing is still missing on the scan tool. 3rd trip light is on cyl. 4 miss again only. Rechecked for vacuum leaks followed vac diagram no leaks. sprayed carb cleaner around hoses plenum none found. Im wondering if the coil pack is going bad.... Help anyone...