The adjective spasmodic means “of the nature of a spasm; characterized by spasms or convulsive twitches; marked by jerkiness or suddenness of muscular movement.”
In medical terms, a spasm is a “sudden and violent muscular contraction of a convulsive or painful character.”
Both words are used figuratively.
For example, inconsistent or occasional efforts at political reform are said to be spasmodic in the sense of:
Occurring or proceeding by fits and starts; irregular, intermittent; not sustained or kept up.
A speaking style, such as that of Milla Jovavich in The Messenger, is spasmodic in the sense of
Agitated, excited; emotional, high-strung; given to outbursts of excitement; characterized by a disjointed or unequal style of expression.
Here are some examples of the use of spasmodic on the web:
To a roomful of adoring gays, a spasmodic Pelosi said, “Don’t ask don’t tell…will be a memory come Christmas.”
Spasmodic reform and a little effort before elections will not accomplish much.
…the electoral parties, always embarked in the rhythmic and spasmodic succession of seductive promises…
Violence still occurs, but it is spasmodic and much reduced.
…there is the most horrendous spasmodic knocking noise coming from inside the [1.2 nova] engine].
Sometimes it’s possible to reach for the word spasmodic and not quite make it, as in this complaint from another unhappy car owner whose Mini-Cooper engine had been knocking for weeks before the incident described:
…[the man at the dealership] said that I could have the vehicle towed in at my expense (his estimate was $600 for the towing). I declined due to the cost and the spermatic knocking.