EniGmA1987 wrote:or maybe the ground is not tightened enough. There are lots of people here who thought such a small thing couldn't possibly be the issue, yet tightening the ground just a bit tighter fixes all sorts of strange electrical problems.
Allow me to second that. I had a similar situation. The cables were actually tight against the battery terminals. However, the ground cable was not seated flat against the battery itself. I had to hold the cable down flat against the battery while re-tightening completely and all the weirdness stopped. I never would have believed it if I hadn't seen it for myself. Just take a few moments to check ALL connections and be sure they are fitted properly AND very tight.