I'm not familiar with the 7M, but typically coil packs can get moister in the contacts or even inside them though small cracks which will cause them to short out until they dry out and run fine again. If they run a coil-on-plug setup like some of the Nissans this is also the case with moister down inside the rubber boot that covers the contact between the plug and the coil.
What you could try, once they are dried out and the car is running fine and your about to park it up for a couple of days, spray some water dispersant spray into the contact areas and around the coil/s. See how that goes.
When you first start it up after it's been sat for a while and immediately starts missing, are there any signs of what it could be causing the problem? Ie. Smoke, if so, how much & what colour. Fuel smell?
Quite often this problem can be due to a leaky injector but i dont think it would cause a problem to the extent your making it sound like.
When you took the spark plugs out, what did they look like? No cracks on their casings or anything like that. How were the tips? Wet? Black? White? Or a browny-black on the base going to a whitey on the tip?
Do you loose any water?
Have you had a compression test done?
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