I tried to give a jumpstart to a neighbor. Connected both sets to both batteries. I turned the key and my car would not start even though the lights showed a full MB battery charge. Checked the fuses and moved them around and car started. (Told the guy to get a new battery by the way.)

A day later same nonsense. Same remedy. Tonite it only started when I moved the battery cables around.

The starter was replaced about a month ago (rebuilt).
Any ideas before I talk to the repair shop?
Thanks,

Juan