Sorry to drag up an old post but I have been meaning to give an update on what had happened to my bike so hopefully you guys won't go through the same thing I did.
Not too long after I had replaced the first bad spark plug a couple weeks later my bike begin began idling poorly had no acceleration and the cat turned bright red after warming up. Just like it did the first time except this time it was a different cylinder. so I pulled the spark plugs again. I check the manual and the spark plugs that where in it were not the standard ngk iridium plugs, they were the same part number minus the i for iridium. And then it dawned on me what happened, when I was in the process of getting my bike smog tested for Japan the mechanic had mentioned that he replaced the spark plugs because the exhaust gases were too hot. Well in a nut shell I blew two spark plugs within a month and I contribute this to the plugs not being the correct iridium one listed in the manual. I ended up putting the right plugs back in and haven't had a problem since.
Good information. I still wonder why the manual calls for replacing iridium plugs every 7500 miles. I had the dealer do mine at 30,000 miles at the same time as my first valve check. The OEM iridium plugs looked fine after 30k. But I figure if I'm doing the labor for a valve check might as well replace plugs then. BTW: I put iridium's in my DR650 as well. They last a long time.