My 99 gt which i have owned since new started leaking at the thermostat housing, which has just been set up at a recall at ford. Several months ago i switched out the intake for one with the aluminum water jacket and thermostat housing. Also i have added the underdrive pulleys new plenum and throttle body. I have got this skip in #4 now that i can't seem to diagnose. New plug, new coil pack, replaced valve springs, compression test is good. I replaced the old pulley/damper on the crankshaft, still no help on the skip. Any suggestions what else to check?: