Check out www.ngk.com for spark plug info.

 

As for the rev-limiter, I paid $4 bucks for the part, and Yamaha paid the labour, so I won't complain too much for that. I still found it cheap from Yamaha. I spoke to a tech rep from NGK spark plugs and he said the that the spark plug is "fouled" due to the carbon build-up. He suggested that the carb be re-jetted again, it's running too rich and that causes carbon to build up on the plug, which in turn weakens the effectiveness of the plug itself. He said that the new plug will work fine (no stalling) until the carbon starts building up again on the new plug.