I have 2003 5. 9 non HO with about 150K on it. Sometimes my idle is sporadic it will move from 700-750 bouncing up and down just enough to move the tach. Also just before I come to a stop the truck down shifts to first and the rpms drop well below idle like it is going to die and bounce back up to idle speed. It has also done this when the truck is cold and in park. I will start it and it will idle fine for a few seconds and then act like it is going to die. It did this about four times in a row last night. My mileage has been slowly decreasing and I have hit an all time low of about 12 miles a gallon. No check engine lights, no codes, but the check gauges light will kick on and turn off again so fast you barely will notice It when the truck seems to be running fine. Please advise