2003 GMC Yukon Catalytic Converter

I have a GMC Yukon where I replaced one of the catalytic converts last June. I have been having the check engine light on which is reading as a mis fire codes. I brought it to a mechanic who confirmed that it's misfiring, but could not pin point where the misfire was coming from. Now I am hearing the same noise that I was before I got the catalytic converter repaired last year. I was wondering if I was to get a tune up, would this resolve the issue? I know when I had the catalytic converter replaced they told me that I had an running problem with the electrical. But they couldn't tell me what the running problem was. Please let me know what I can do to fix this issue.