When you fitted them, did you set them to the OE gap of 0.7 - 0.8mm? The iridium plugs have a narrower centre electrode and require a larger gap in order to generate a fat enough spark. If they are on OE gaps, try opening them out to 0.9 - 1.00mm and see if that makes any difference.
I've been running the same plugs as you, (with a 1mm gap), on my Evo for over three years with no problems.
Some more information here at posts #4 & #8:
http://www.ukmonster.co.uk/monster/s...ad.php?t=54154