2003gtstang

posting this for my buddy (85 5.0 GT) his cobra is running like ****, spittin blue/gray smoke. the car is breaking up at all rpm's. a plug blew through the head orignally, the previous owner helicoiled it and the plug blew out again. so since he has a year warranty on the motor, he brought it back and the guy and he helicoiled it again and now it runs like ****. when the plug blew out this previous time, he limped it home on 7 cylinders with the plug/coil out obviously. but like i said, now it runs like **** and spits out blueish/gray smoke. any ideas? thanks guys