I'm getting a P0306 trouble code. The car is running rough at idle, seems to run a little better when you increase the throttle, but still seems like it might be missing. I checked the coil, swapped it out with another coil, both tested okay. Checked the wires which were replaced about 6 months ago, replaced all 6 plugs, checked the injector resistance and all were okay, pulled the injectors and cleaned them and still have the same issue. I replaced the upper intake manifold at 58,000 miles when it ruptured and dumped coolant into the engine. Also replaced the lower intake manifold gaskets. At that time I had replaced the plugs. The car has 67,000 miles on it now and whe I just pulled the plugs they smelt like old gas, yet the car is driven daily about 50 miles. Any ideas where my problem is?