misfire help

99 cobra has a misfire on cyl 8. changed the plug and changed the coil pack
5x already. sometimes when you change the coil pack it will run fine for a day or two then it goes back to missing again. a week ago it threw two codes for misfire cyl 8 and igntion coil malfunction cyl 8. now it throws the check engine light but it says no codes found when you put a code reader to it. im at a loss guys any help would be appreciated.

no electrical mods. one day it had a back out plug that busted the coil pack so i replaced it. then it started missing real bad. i have been through 3 aftermarket replacements and it would fix it for a day or so then it go to missing again. now im on coil pack number 5 and now the dang thing dont even fire on that coil pack. i unplugged the one coil and the injector to that cyl and the car is still missing.