I'm having the same problem, and it is causing a lot of fuss for us as our members can not sign up for constantly being told that the captcha is wrong, when it is actually correct. I have tried turning it off, waited a while, and it is becoming quite a pain in still showing and putting members off. While i personally like the captcha, we need it turned off!
Any help would be great. our board is at http://541948.xobor.com/
Thanks