I have a 98 explorer sport 4.0l sohc engine. Has trouble starting. This is a new problem only 3 days ago did it start. It cranks but doesnt want to start and when it does the rpms are too low to keep it going. After about 5 or 6 tries it finally starts up. Changed out the spark plugs, wires, coil pack, battery, fuel filter, checked all fuses and relays, all hoses and wires, no vacuum leaks, intake gaskets are fine...anyone have any ideas on what may be causing this?