Honestly, the engine management system on these car is so horrible that inexplicable changes in power and gas mileage are typical. I've been driving my car for 70,000 miles, and I've had power/mileage all over the map despite very consistent driving. My car has terrible power right now for absolutely no reason.
Are any of your brake calipers dragging? The only definitive causes of my power/mileage losses were when each of my front calipers started to drag. You would think a dragging caliper would be obvious when you're driving, but it isn't. Go for a drive using minimal braking, and then compare the temperature of each caliper by touching them. If one is much hotter than the other, then your caliper is sticking.
Are any of your brake calipers dragging? The only definitive causes of my power/mileage losses were when each of my front calipers started to drag. You would think a dragging caliper would be obvious when you're driving, but it isn't. Go for a drive using minimal braking, and then compare the temperature of each caliper by touching them. If one is much hotter than the other, then your caliper is sticking.