2000 Jaguar S-Type 4.0 Problems

Hello. I have a 2000 Jaguar S-Type 4.0 with 58,000 miles. A while ago, the car started idling a bit rough at times. With this new problem came significantly decreased gas mileage (from 25 mpg to 13 mpg), and decreased power. When the car reaches above 40 mph, it stutters and vibrates. The check engine light also came on. I was told by various other 2000 S-Type owners that the valve cover gaskets can fail enabling oil to flood the spark plug wells and the ignition coils. When I pulled off the valve cover gaskets, I found that oil did saturate the spark plug wells as well as the ignition coils. I replaced all the gaskets as well as the spark plugs. The ignition coils were cleaned up and reused as they still appeared new and undamaged. After all was put back together, my problems still persist as if I did nothing. I took my car to Autozone to get a free engine diagnostic and the codes indicated that the problem could be a faulty Idle Air Control Valve or the Oxygen sensor. The codes were then cleared and the check engine light went out. When the diagnostics were run again, only the oxygen sensor was read as faulty. I replaced the oxygen sensor in the location specified but again, it was as if I did nothing at all. Before investing in a new idle air control valve I wanted to make sure this is definitely the problem. I do not want to take the car into my Jag dealer as they will charge me $220 just to look at my vehicle. If this is something that I can fix I would much prefer to do it myself. Any help is greatly appreciated. Thank you.