The problem may be caused by a twice loaded jquery library. First, on a page, and second time via ajax. You should not load it on ajax request. See a related topic.
I figured out the captcha problem: loading the contact form via ajax, the yii generated id of captcha image was not unique, because same id exist in the main page in background. I handle the problem by specifying the ids of button and image manually.
I think it would be nice if yii add more uniqueness to generated ids via random numbers or something.