I have a 98 3500 5.9 turbo diesel. In the morning or after sitting for a long period of time, I walk out to truck put key in let it warm up, then turn the key. Fires right up purrs like a kitten! So I drive to the store turn it off and shop say 20 min. Go out to my truck and do the same procedure to turn it on except now it has to crank for 10-15 sec to turn on and some times does not want to turn on at all. What could my problem be? Any help would be great thanks!