the possibility of Shaker Dog Syndrome. It is a genetic defect thought to be a defect in a neurotransmitter, and can appear in Malts from 3 mos - 3 years of age. It can range from mild (such as yours) to severe where it may last many days without treatment.

Cortisteroids or tranquilizers such as benzodiazepines are given to provide better control. You might consider asking the Vet for a low dose tranquilizer to see if it helps prevent it.