If I had to bet, I'd put my money on the regulator. USUALLY a generator either works or it doesn't; the kind of problem you describe sounds to me like the contacts in the regulator relay are getting burned or just worn out. You can burnish them with a bit of paper or thin cardboard between the contacts, or you can get a special file for doing that. Check the adjustment, too, of course.
A lot of the new components from the usual suppliers are not good quality. It might help to get an original, Lucas one, even if it's used, and see if that has a better lifetime.
Oh, yeah, be sure to disconnect the battery when you work on the regulator, or anything in the charging system, for that matter.