2004 Kia Optima engine stalls

After car has been driven more than 30 to 40 miles at one time without cooling down it stalls when coming to a stop or loses power if going 45 miles per hour or less. Sometimes bucks while going over 45 miles per hour after driving more than 40 miles at one time.
Not showing anything wrong when put on diagnostic tester.
Mechanic has changed spark plugs and it still has the problem.