Your plugs are fouling, are they all fouling, or just ones from certain cylinders? Like Zef said, you may have the spark plug wires running to the wrong coil packs. If it's cylinders 3 and 4 you're receiving misfires from, those 2 may have the wires switched. And like others said, haven't heard good things about MSD coilpacks. Some though few, have run them with no problems.