Don't know if this could be a factor or not. My idle is pretty low, but haven't had any issues. Was having a problem with high idle etc and the car always wanting to crawl forward. Turns out the linkage where it pivots to pull the throttle cable was really stiff and stuck a bit. Lubed it up and haven't had the problem since. Not sure if this is a factor for you, but worth a look see. Haven't had a problem since.
As for mileage, I'm by no means an expert, but with my 4.0, we replaced all the spark plugs, fluids, ran higher octane gas for one full tank, new oil filter. I got approx 400km+ per tank, and in washington state passed emissions with flying colours. I also wonder if the mileage is related to the high idls as well. Just a thought.