I don't think it's unethical as long as--as you mentioned--you aren't doing it all the time! Some of my early articles got barely any recognition because I had yet to gain an audience. When republishing old content, I'd like to think most writers would take time to make improvements from what the piece once was too. The couple of times I've posted old content, I've worked to still create a new-ish version of the older article. But that's just me.