2005 Kia Sportage need help

My better half has a 2005 Kia Sportage EX v6 with 109k on it. Starting last summer when the car was driven long distances; on hot days with the AC on and off; and the cruise control on and off, the engine would start to stumble; lose power; and the check engine light would come on. I pulled the codes and found po420; po430 bad converter and po305 misfire cylinder 5. We have had the fuel system cleaned twice and the wires and plugs replaced. I also add Chevron with Techron to the fuel tank every other fill up. (She runs her car to E a lot). Know matter what we do the check engine light still goes on and off whenever it wants to. I don’t think the converters are bad there is no smell and the car runs good when the light is off. We have had the car at 3 places (1 dealer and 2 independent shops) they only give us guesses not answers. We don’t want to throw money away on guesses. Any ideas?