My previous girl, Elka, had the head tremors. she developed them at around 6 months, after her spay. They would typically manifest if she was super anxious about something, or if she REALLY wanted something (she did it once while we were carving the turkey at Thanksgiving), and the tremors would typically subside if we distracted her with treats, had her do a few tricks, etc.
When I consulted the vet, they'd never heard of it (it would be really great if I could find a local vet who'd ever heard about Dobermans like, ever) and just called them "idiopathic head tremors", but had no treatment or management suggestions. Since it was so minor and intermittent with Elka, not even daily or weekly, I never pursued it further other than as an "oh that's weird". Ulrike has not manifested it, that we've seen, and she's 10 months old now, not yet spayed.