My Jag has the same alternator. The 6RA relay went bad, despite (or because of) being a new replacement. It was arcing and causing the alternator to overcharge. I replaced it with a modern relay as they’re much cheaper, and so far much more reliable. The 6RA isn’t particularly waterproof, and mine was a bit corroded inside when I took it apart. You could try a modern substitute to see if that cures the problem.