My ZETEC did the same thing a couple of months after I replaced the plugs and wires (Bosch wires and Autolite double plats from - you got it - Autozone). I pulled the plugs to look at them and noticed two of the plugs had carbon tracks burned down the porcelain where they had been arcing inside the plug boot. Scraped the carbon off and put some insulating and waterproofing compound on the plugs. I also put some of the compound in the plug wire boot before putting them back on the plugs. Been running great for a couple of months now. You might wanta look at your plugs.
