re: Chevy/GMC mechanics

You could have cable problems. As I said check both ends of each cable. It sounds like a connectivity problem. If there is corrosion between the cable and the battery post and you try to jump it you are connecting the jumper cables to the outside of the battery cables and not the battery post/terminal itself. If it is a side post the corrosion is hard to see. If you are not sure what you are looking at bring to a good shop.