Your negative terminal is either loose or the cable is corroded internally. What's happening is the ground connection is no longer clean and has becomea point of high resistance, not allowing you to start the car. If moving the wire around sometimes gets you to a point where it will start, either the terminal is loose or the wire inside is nearly gone.
Replace the negative terminal and length of wire from the terminal to the ground point on the body with AWG 4 wire or better, and your problem should go away. Happened to me a year ago. I would move the wire around and green dust would spill from inside the wire; the copper in there literally rusted away.
Here's what I did:
I cut the old cable off and used the connection point on the body for the wire I just cut off. Haven't touched it since, haven't had an issue. Hope this helps!
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