String Trimmers

I am currently working on a Husqvarna 128LD string trimmer. It will start but then begins to miss...definitely an ignition problem. I installed an in line spark tester and the unit ran fine. The trimmer had a CJ8Y spark plug in it. I then installed a RCJ8y spark plug and again the trimmer ran fine. I thought the "R" in the Champion spark plug was for radio interference. Why does this 128LD run fine with a RCJ8y and miss with a CJ8y? I thought that I could possibly have a bad ignition module. So a new Husqvarna ignition module was installed and the air gap properly set. But I stiil experienced the same problem.
Thanks for any information.