NGK?
is that what comes in these from the factory? if so then i guess we can say it probably isnt the plugs as long as they are gapped correctly.

So when it does run it runs good but when it acts up nothing will get it to run. Replacing the crank sensor fixed it in the past, now it has a metallic sensor. Have you gone back and rechecked this sensor to see if it is failing? or checked the cam sensor? (i'm sure you have)
Have you checked the timing to make sure it is staying in time? It might be a key or something could have sheered and is slipping causing the engine to get out of time occasionally causing this failure. It seems unlikely but i'm thinking outside of the normal box now.
As long as you know you are getting spark, air, and fuel then it would seem likely to look at other possibilities that could cause this.