ei8ball

I'm having a slight problem with my 99 GT. I first had an error code of P0304, which indicated a misfire of the 4th cylinder. I changed all the coil boots as the 4th did have some broken rubber. That solved my problem for a little while until I kept getting P0307. I took it over to my mechanic, and he said that the coil was not producing enough voltage, so it was replaced. He did a compression test and said everything was normal. He told me to run it a few days to burn the excess fuel out of the cylinder. After driving for a while, the check engine light started flashing again and then stayed on. While accelerating, at times the car would lag back and then come back again. I have since replaced the fuel injector and the plug to that cylinder and still cannot get P0307 to stop coming on. I'm running out of ideas and getting more than a little annoyed shelling down cash to fix this problem. Any ideas or help would be greatly appreciated.