Apocalypse

I I have a 92 XJ sport 4X4 4.0 and I keep getting a code 27 ( injector not firing when told) that doesn't stay active all the time and when I clear the code it goes away for about a week. When the check engine light comes back on my Jeep starts acting like it's missing especially in 4th and 5th gear. I have replaced fuel filter, tested the injector with a node tester, taken off the injectors and cleaned them individually, tested the resistance on the injectors and they seem to be good also tested the power to the injectors and seem to be good there as well. The injectors are the upgraded ones for the XJ and are yellow with part number 028155700. One more thing is that it is running what looks like really lean on all 6 cylinders as the plugs that I replaced have turned very white. Any help in diagnosing is greatly appreciated. All help would be greatly appreciated.