I’ve had this problem for a few years and no one can figure out what is wrong. Here’s the jist of it…
After driving for a lengthy period of time (let’s say over an hour) and the engine is good and hot (high temps outside only make the problem worse) the Kia will start to act like it is going to stall out, the RPM’s drop drastically and only go back up when it wants to. It has died on me a few times (when idle, in the middle of an intersection.) Sometimes even when I am driving on the freeway the RPM’s will drop and I won’t be able to accelerate until it kicks back in.
An example of what it feels like is a kink in the hose that gets the fuel to the engine. Then all the sudden it will let up and lunge forward letting all the fuel to the engine at once and causing a big acceleration.
I tested it today after it had been acting up a lot, I had about 1/4 of a tank of gas and pulled over to fill up. After I filled the tank with the highest grade fuel it stopped hesitating and acting like it was going to stall out. I don’t think it has to do with the fuel grade, but I do think it has something to do with the fuel tank. I even sat idle for about 30 minutes and the RPM’s didn’t freak out once.
I’ve had my Kia at a number of places, most recently the mechanic didn’t find anything major wrong and the Kia never flinched for him.