Here is another interesting observation of the strange aspects of PDS. Per Alm pointed me to a study of the menstrual cycle, showing more stuttering during the premenstrual phase! This clearly points to neurotransmitter (brain chemicals) levels in the brain affecting fluency. So PDS cannot really just be psychological, because the menstrual cycle is related to changing hormone levels?
Silverman, E. M. (1975). Speech fluency fluctuations during the menstrual cycle. Journal of Speech and Hearing Research, 18, 202-206.
Abstract: This study sought to determine whether changes in disfluency characteristics accompany biological and emotional changes during the menstrual cycle. Four extemporaneous speech samples were collected from each of 12 women, one at ovulation (when the average woman experiences her greatest feelings of self-esteem and self-confidence) and one at premenstruation (when she experiences a significant increase in anxiety level) for two consecutive cycles. The mean total frequency of the women's speech disfluencies produced at premenstruation was significantly greater than the mean total of their speech disfluencies produced at ovulation. The disfluency type that accounted for much of the difference in total frequency of disfluency between the two cycle points was revision-incomplete phrase. The tendency for these nonstutterers to produce more speech disfluencies at premenstruation than at ovulation is similar to a tendency we have observed in stutterers
Silverman, E. M., Zimmer, C. H., & Silverman, F. H. (1974). Variability of stutterers' speech disfluency: the menstrual cycle. Perceptual and Motor Skills, 38, 1037-1038.
Abstract: Studied whether women who stutter produce more speech disfluencies at premenstruation (when anxiety level has been reported highest) than at ovulation (when anxiety level has been reported lowest). 4 speech samples were collected from each of 4 Ss, one at ovulation and one at premenstruation for 2 consecutive cycles. Results show that Ss produced more disfluencies premenstrually.