I'm a speech and language pathologist (speech therapist). A lisp is produced with sounds like s, z, ch, sh, and j (jelly). Difficulties with the R could be called a distortion or a simple substitution, for example, saying a W instead of an R. Stuttering is not caused by such a substitution. If you were meant to say W instead of R, it's unlikely that would cause a stutter. Many people with a stutter have particular sounds they stutter on.