Just to concur with what you say, i spoke to the guys on the NGK stand today (very helpful) about problems some members have encountered, they say it is down to the wrong plugs being fitted
I explained to them about the problem i had with a Laguna V6 that kept dropping a gear (auto box) every now and then, we looked up the part number to find it was right on the change. The NGK's fitted were the early type, it should have been fitted with the later platinum type which Renault sent me. It's all to do with the resistance of the plug type