the idle problem sounds like the IAC( idle air control valve). its on the back of your intake manifold. its only 2 bolts to take it off. all you do is clean out the screen on it and you should be good to go. also check our your throttle body. it wont really cause any of the problems you're talking about. but it may help with the idle. just look and see if the butterfly valve is dirty. open it up and check the opposite side. should help with gas mileage also.