I'm not too familiar with the intricacies of Extralite hubs, but campy lockrings have a pretty long threaded section. You might want to make sure the first threads aren't rubbing on some stationary seal when tightened. If they are, when the freehub is turning, there will be an unscrewing force applied.
Only once in many years have I come across a guy with a chronically loosening cassette lockring. It was a Shimano XT. Tightening it didn't help. Neither did overtightening it, neither did Loctite. And neither did a new lockring. It was decided that the serrations on the first cog were compromised, possibly from the very first loosening. New cassette, no more problems.