Misfires

Mercury Grand Marquis 2011, with 140k km mileage.

Few months back it has lost a huge amount of power
suddenly and with no previous problems. across all gears
(Engine weakness) tried to diagnose the reason but
couldn't find it. About a week ago an engine light came on. I
was happy. The code is a misfire code. The car misfires for
about 2-3 seconds then goes back to normal only during
idle. And not always, it happens randomly and only during
idle. Usually during a cold start. The misfire is not on one
cylinder. Each time it happens in a different cylinder. Tried
to change the spark plugs, changed 3 ignition coils, good
fuel pump, good fuel filter, good air filter, cleaned intake
manifold and cleaned MAF sensor. The problem is still
there and a misfire happend at cylinder 3 where the new
ignition coil is. So problem is not the coils. Drove the car up
to 150km/h and it could still go for more. So I'm guessing it
is not fuel related. Also the startup is smooth and healthy.

If possible, give the probable cause of this. Thank you and
have a great day.