Ok so my car runs smooth when cold after im parked for over 15 minutes with car on temperature goes high and then as i drive away it starts cooling down stays in middle but has rough idle i did a radiator flush put new coolant and water it still does the same thing but only when im parked for over 15 minutes i checked oil and no water mixed in there any help would be great? Its a 2003 4.6 mustang gt automatic completely stock