The subject of stuttering has had some prominence recently as a result of the film "The King's speech"

The Welsh for a stutter is atal dweud,  to stutter is siarad ag atal

Roedd y Brenin yn siarad ag atal yn ystod ei araith

"The King was stuttering during his speech"

Dw i'n siarad Cymraeg ag atal achos dw i ddim wedi ymarfer y gwersi SSiW bob dydd(!)

Atal is a useful word meaning "to stop" (in the sense of "prevent") and is followed by rhag:

Naeth y tywydd atal y gêm rhag fynd ymlaen

"The weather stopped the game (from) going ahead"

Hercian is another of those nice multipurpose Welsh words with several related meanings. It can mean to hop, or limp or stutter. One can see a common thread to all the meanings.