I obtained a new NGK iridium plug of ebay and once fitted it into my XTR I noticed the bike did not run as well as it did when running on standard and the fuel consumption increased.
I check up on the NGK web site I found that the plug I got of ebay was a counterfeit, if you have the old box the plug came in, the NGK web site will show what to look out for and how to spot a counterfeit.
Since then I have fitted another iridium plug but this time supplied by local motorcycle shop and its happydays are here again.