Both my CTD are having the same issue. After starting the engine idles well for about 5 seconds and then it stumbles. If I get on the throttle it runs rough for about another 5 seconds and after that runs smooth. It does not seem to get any more air if ithe truck has been sitting for an hour or a few days. And they both only do it when the fuel tank is under half full. When the tanks are full no issue. What gives?