Is this on the /r/ sound in the beginning of a word? A lisp occurs on an /s/ or "th" sound (sounds that are in the front portion of the mouth) typically so I would not believe that you are avoiding a lisp by stuttering. from my experience, /r/ is not a typical sound to stutter on, but stuttering is varied and unpredictable. Without hearing your speech, I would suggest trying easing into the /r/ sound that you block on by lightly voicing a vowel beforehand (i.e. instead of saying "rabbit", say "errabbit") to open and loosen your vocal tract to avoid the blocks.