how have you checked the coils? by removing one at a time to see which one doesn't change the misfire?
It is possible to have a perfectly fine coil but the signal to it (from ecu) is bad. This may explain your intermittant problem (dodgy connection in ecu.
Once I spilled 10ml of oil outside the filler cap (on my 6 cylinder) only to find it had half filled a spark plug recess causing that cylinder to misfire. Once i'd removed the oil it ran fine but a week later the problem returned as I had "surged" the ecu (with the oil) destroying the signal to that particular cylinder.
All my coils tested fine, it was the ecu that was fappin me around.
i'm inexperienced here though so I'd listen to the others.