Peter, which NGK were you using? I kept fouling the 6, but the 5 seemed to be much better. They did still foul occasionally. And I found a good scrub, then a rinse with thinners, and cooking on the stove for 5 minutes sorted them.
I am now using the 3-pronged Bosch, they are very hard to foul, though the PI in the alps seemed quite adept at it.