'97 MGM intermittent misfire issue

Hello, everyone. New to the site. My Grandad has a '97 Mercury Grand Marq' 4.6 with only 26,000 on the odometer.

For quite a while now (about 5 years or so), it has had an intermittent issue where it will just start misfiring and shaking for no reason. Just going down the road and all the sudden it starts shaking and the check engine light starts flashing for misfire and the A/C shuts off because of it. I'd say it happens maybe once a month and then all of a sudden it will clear right up and run perfect for another month. Here is what we have done to it so far:

New OEM spark plugs
New OEM coil packs
All new OEM plug wires

None of it has made any difference. It's not a big problem since it's intermittent and doesn't last long when it happens, but I just haven't seen this happen on any other Crown Vic'/Grand Marq's. But if anyone could give me some insight on what may be the issue, I would really appreciate it! Thanks.

i had this problem with my 2006 besause their was a small crack in the intake and some of the anti freeze would get it the spark plug well and cause the car to miss

Thanks for the reply! I will look into that and see if any of the wells have any coolant in them. Did yours seem to only miss on occasion? I can't seem to figure out why it only misfires sometimes but not often.

yes it was random but it eventully got worse

Well, we went and checked all the wells yesterday. No coolant is evident in any of them. Coolant is not a drop low either. Anyone else have any ideas? Kind of stumped here. Another thing I noticed recently; when it starts missing, I can pick up the #7 wire off of the coil pack and snap it back on and it will stop missing immediately. Really weird.