I just bought I 98 5.9 cummins turbo diesel truck. When you walk out to the truck first thing in the morning or after sitting for a while. I put the key in give it some time for flow plugs to warm up and then it fires right up really quick! Cool no problems! Now if I drive to the store turn it off shop for idk 20 min come out it takes a while of cranking to turn over. Some times does not want to run at all!? What could be the problem what do I need to do? Any help would be great thanks