I used to work at my friend's motorcycle shop during college. One time we had a batch of ND plugs that drove us nuts. The plugs would work OK cold, but would start misfiring after they got very hot. The misfire did not manifest itself during a short ride after a tuneup. It would happen after the bike's owner would take an extended ride. At first we replaced the usual suspects: condenser, coils etc. We also replaced plugs, but used some from the same batch, so the problem continued.
We finally discovered the plugs where bad when my friend installed a set on a personal bike, and rode home. He had not changed anything else, and the bike started misfiring on the freeway. At home that evening, he replaced the new NDs with an old set of plugs. He rode back to the shop the next morning with no issues.
The ND distributor reimbursed us for all the boxes of plugs we had recently purchased. They also sent the plugs back to ND for analysis. We couldn't afford another situation like this again, so we quit using ND for a while, and used NGK and Champion exclusively. Eventually, we went back to using ND about a year later, and never had another problem.
