Try running it when it's dark outside and see if there's a spark going to the engine somewhere around the coil pack. It may be some broken down insulation on the pack. However, if it's down just above the plug you might not see it. You could try pulling the coil and look for bad insulation. This doesn't make perfect sense to me with the symptoms you're describing but you might give it a look. I don't understand why disconnecting the injector stops it unless the PCM stops the spark when it senses a failed injector.
2007 GT CS, black,FRPP CAI