G.B.
My 3 year old daughter has experienced that a couple times too, beginning around age 2. She is pretty articluate for her age and I was told that she is processing information in her mind much faster than she can get it out. I noticed that when she experienced stress (potty training or consequences for misbehaving), it was more noticable.
My husband and I both found that if we called attention to it, asking her to stop stuttering, it worsened. After we agreed to ignore it, it completely went away. We often had to "gently" remind family and friends not to call attention to it either.
Good Luck!