3.9L V6 engine. Bought it about 2 months ago (salvage title) and it drove great for the first 5 weeks. Started to misfire and check engine light came on with codes 300,303,306. Replaced the plugs. wires, and coil pack, still misfiring. So bad now that it is barely driveable. Replaced fuel filter and air filter....running out of ideas. Any help!?!