I have an 08 suburban ltz with 5.3 flex fuel. I have an issue with slightly rough idle goes from 500-700 while idling and it’s not all the time. I have replaced plugs and wires because of misfire codes. The exhaust manifolds were replace because of state inspection. The upstream o2 sensors were replaced after the exhaust manifolds. No codes for anything. Did an idle/fuel relearn. Also had smoke test done couldn’t see any leaks. I’m beating my head against the wall here any suggestions would be greatly appreciated.