I have an idea of what could be causing the drop in idle but I'd rather someone else answer that as I am not certain in my answer.
As for improving gas mileage you will want to change your driving habits. It's the biggest factor on gas mileage. Stomping on the gas is a lot of fun but when you don't really need to, like merging on highways you're just using a lot of fuel for something else.
Other than that regular maintanence. Clean air, fuel and oil filters. Correct, consistent tire pressure can also help. Although your idle problem may also be affecting your gas mileage.
