Last Tuesday I installed a bottle of Chevron fuel injection cleaner. The 3.3 was running fine before I did, but I thought I was doing some preventative maintenance. About 10 minutes of running, the engine started missing. This continued, even after I drove over 100 miles yesterday. Today I pulled the front 3 spark plugs, and they are Autolite Iridiums. Two were gapped at .050, but one was a little loose, gap wise, so I re gapped it. I put a little anti seize on the threads after wire brushing them, and swabbed some dielectric grease on the insides of the plug boots and reinstalled them. It ran no better. This van has about 200K+ on it and has ran well, I get around 25+ mpg on the highway. Any ideas what could be causing this?