Spasmodic dysphonia, also known as laryngeal dystonia, is a disorder in which the muscles that generate a person's voice go into periods of spasm This results in breaks or interruptions in the voice, often every few sentences, which can make a person difficult to understand. The person's voice may also sound strained or they may be nearly unable to speak. Onset is often gradual and the condition is lifelong.

Source : Wikipedia

The cause is unknown, but is thought to be related to abnormal functioning in an area of the brain called the basal ganglia. Stress, infection and genetic factors may be triggers.

Further info : US National Institutes of Health (NIH)