2002 Yukon

I have an 02 Yukon with about 50k miles. The service engine soon light came on (Flashing at first then went steady). Plugged a computer in and got code P0305 (Misfire #5). I changed all the wires and #5 plug and coil. Light continued to flash after reseting computer. Originally I could not feel any difference in the truck. Then it got progressively worse(Truck shuttering and very rough idle) I brought it to my local mechanic who put his computer on it and found multiple misfires on almost all cylinders. He checked the fuel nozzles and systems and it checked good. Has anyone have any idea's on what this could be or better yet has anyone had this problem. I am thinking it's the computer or a faulty ground. Thanks for your help