Miatas eat plug wires. Change those first. A lot of people swear by NGK Blues but I see them fail all the time too. My Magnecores have been on my Miata for 8+ years and are still going strong. They do have a lifetime warranty if needed but they are not TCB approved. Track Dog has the Magnecores for $74.95 versus about $30 for the NGK online. You get what you pay for I guess.