2002 Buick Park Avenue deteriorating mpg, 3800 series V6

My fuel mileage has suddenly decreased by 10% and sometimes there is a rough idle upon ititial start up that clears up as soon as the engine is reved. This rough idle seems to occur when the car has been parked with the front wheels cocked to one side but that could be coincidence as I can't imagine a connection to steering. There is also an ever so slight miss that can be felt in the steering wheel. A person not used to the car wouldn't notice it. Average mpg noted on the computer is down from 27.5 mpg to 24.5 mpg and the majority of my miles are highway. The vehicle is serviced on a regualar basis. I run mobile one synthetic and oil useage is negligible between oil changes. I bought the car 2 years ago at 50,000 miles and to my knowledge plugs and wires have never been replaced. This is my second Buick with this engine and in my book it is hands down the best full sized, domestic that I have ever owned in terms of ride, mpg and repairs. My first one was a 97 model running strong at 180,000 with 0 oil consumption. I can't say enough good things about this make.