I have a 2007 F150 5.4L. The engine will sometime stumble when it idles. Seems to happen more often after the engine is up to temp and has been driven a while. Come to a stop and the idle will drop to almost a stall and sounds like it's going to die a horrible death. Shift to neutral and give it a little gas and everything is fine or two foot it with the break and gas and things are ok. I've had similar problems with other cars and it was the idle air by-pass, but I don't think this engine has a by-pass valve. I have check vacuums, air filters, cleaned throttle body but nothing seems to help. Any suggestions?