Ironically, I've heard it targets longstanding members more than new ones. I'm afraid this forum is being hosted on the cheap, and we're suffering the consequences.
Yes that's the other rub - It seems to attack when I've typed a lot, taken a long time about it, and included multiple quotes, in other words at exactly the point when I'm likely to have done a lot of work. It couldn't be more annoying if it had been deliberately programmed that way.
One wise precaution is to copy any such post to the clipboard before hitting the Submit button. If you forget to do that and the capcha comes up, be sure to hit the back button, and with a little luck your work will still be there. In my experience, once you attempt to solve the capcha, some or all of your post will likely be removed whether or not you solve it.
I guess I'm lucky - I know of another website that uses the same crapcha thing after I've been browsing there for a while, with the one small problem that it fails to even display the puzzle in my browser. Luckily it's not the kind of website that I'd want to post anything on, so I don't lose any work, and the solution is to simply abandon it for a while and go back when it's simmered down.