have you tried a noid light to test each injector is firing. I had one that was doing that. the noid light stayed lit the whole time on one of the cylinders. it ended up being corrosion in the ecu around the legs of the transistors that fire the injectors. I de soldered and cleaned all of them and re soldered. fixed problem. you can buy noid lights at advance I think. its a kit with several for different vehicles. you unplug the injector and plug the light into the socket it will blink when you turn the motor over if all is good. it was the number 6 cyl that did it too. it drove me fuckin crazy. it must have drove a couple other people crazy too, I bought the car on here for like 850.00 and they had put a bunch of parts on it trying to solve the problem.