What to do next ?

What kind of spark plugs did you install? Every once in a while I hear a story on here of someone having similar misfire (or pinging) problems like you are experiencing. Some of those B.S. super duper spark plugs that companies are talking up these days have been the culprit of these misfire woes. Do you have platinum plugs in there? I've heard of the platinums freaking out the EEC-IV computer for some reason. Just something to think about...