Limit the excerpt by a paragraph (not words).

Hello,

You can change what the_excerpt() returns by way of the get_the_excerpt filter:
http://codex.wordpress.org/Plugin_API/Filter_Reference/get_the_excerpt

Example code (not tested):

add_filter( 'get_the_excerpt', 'my_excerpt_filter' );
function my_excerpt_filter( $content ) {
  $content = apply_filters( 'the_content', $content );
  $content = str_replace( ']]>', ']]>', $content );
  $paragraphs = explode( '</p>', $content );
  return balanceTags( $paragraphs[0], true );
}