OK, to start, I had to rebuild the engine in this 1997 Mustang GT. I put all forged internals with a main cap girdle. I was going to use a pair of PI head from a PI swap kit, but the factory they were assembled in bent one of the valves. I had to get the car running so I reused the 120K non-PI heads. The car fired up with no problems, but at idle and under heavier loads it shakes the crap out of the car. I looked at the plugs and all but one shows normal combustion of light brown. #3 plug is perfectly clean and white except for alittle carbon on the end of the threads. I checked the wires, they were locked. I tested the injectors, 15 ohms and the motor stumbles when I unplug them. So what else could it be? The old heads looked fine with no valve leaks. Anyone have any ideas? BTW, I am alittle new at Mod Motors.