In the past week or so I notice if I leave my 98 expo in park or idling at a stop light. It baffle as if its going to shut off. Sometimes I feel it every 5 sec sometimes longer. I changed my spark plugs and wires just over a year ago. I also have an K&N air filter system that was installed about 3yrs ago. I also added a fuel injector cleaner couple days ago and no help. What can the problem be.