I am having a similar problem. I think its a spark plug wire, at least for now. I have changed 02 sensors, MAF, plugs, a/f ratio to no avail. The wires are Ford Motorsports, but after 37k miles, they might be bad. So I am putting the OEM ones back on, one at a time. Well, atleast once UPS delivers them from another CEGer, I threw mine away.

Sold 1998 Contour LX 13.868 sec 1/4 mile time. Two years of rental cars, seriously. Holding out for 07 Cobra Mustang.