07 was the first year for the AFM (Active Fuel Management) system. This is the system that would deactivate 4 cylinders for better MPG. I have heard (on message boards) that the 07 and 08 5.3's, in particular, had problems relating to the AFM malfunctioning. Not sure exactly what the symptoms are (my 5.3 is an 04) but it may be something to check.