Hi Jack,
I had this happen in another vehicle once upon a time.
I found a load that didn't shut off, causing the circuit to complete when I put the cable on the terminal.
Perhaps a switch, somewhere, that is not fully disconnecting?
Putting a circuit load tester between the negative battery terminal and its cable to see if you have some kind of load?
I found it by pulling one circuit fuse at a time to isolate the problem.
But, my problem wasn't intermittent...yikes!
The last similar problem in my Capri was under the carpet alongside the transmission tunnel, a wire that lost its insulation grounding on the tunnel...that was expensive...
Good luck!
Steve