I've got a 2002 Dodge Ram Pickup with the 5.9L engine and 61,000 miles. It recently began running rough (sputtering and backfiring) when idling and the RPMs drop down to about 400. It only happens occassionally (maybe once or twice per day) and lasts for 10 or 15 seconds and then runs perfectly smooth again. When it does happen it almost sounds like it is running out of gas. I do use high grade fuel system cleaner/additive about every 20K miles to try to keep the injectors as clean as possible. Any ideas? Thank You.