02 Optima 2.7L looses power

I have a 2002 Kia Optima with the 2.7L v6 that runs great most of the time, but about once a week it will loose power and run poorly. We shut it down and let it sit for a little while and then it will start right up and runs great for another week. When it acts up it starts right up and idles ok, but has no power and falls all over its self when you try to drive anywhere. Any ideas what to look for or what might be wrong?

Thanks.