mine did the same thing when i first got it. if it was plugged in, it would fire right up, but unplugged it would still start but would struggle a little. i put new batteries in and its been fine ever since.I have an 04.5 3500 and am starting to have some trouble starting in cold weather, now i would assume it was the batteries but i dont have near the trouble if i plug the block heater in. Any suggestions? Ive heard about the Grid Heater? But dont know much about it. Thanks for all your help!