Had a similar problem on a Toyota PU I used to own. Turned out it was a bad positive cable. I'd cleaned both ends of each cable and run through steps similar to yours with no solution. For giggles, I bought new cables and once I'd swapped out the positive cable, the problem went away. I went ahead and swapped out the negative cable anyways. Never had that problem again up until I traded it in.. Might be worth a look.