[Alan Roehrich]

More likely, the stator shaft in the pump is slipping. A sprag usually is either good, or bad, it either holds, or it doesn't. If it works, then doesn't work, then works again, it is most often a slipping stator shaft in the pump. Have the converter repaired because the stator shaft slipping can damage the converter. But it sounds a lot like the stator shaft is slipping.