found this and copied, dont know what breed of dog your is?
Q: Dear Dr. Mike, We adopted what we think is a cocker spaniel (he's quite large, but not fat - about 37 lbs) 10 months ago from from the shelter. Right away we noticed he occasionally had tremors - usually one of his back paws would shaking (thumping?) for about two minutes, and then it would stop. Sometimes it would be his front paw. Holding or stroking the affected limb wouldn't help - but the dog seems completely oblivious to these episodes, so we didn't worry much about it. The shaking episodes occur about once or twice a week. Yesterday, however, his jaw was affected. At first I thought he was chewing something very fast, but when I checked him, his jaw was clenched tight and his teeth were chattering, and nothing was in his mouth. This lasted for about two minutes, and it did seem to stress him out. His eyes weren't glazed over and he didn't lose any other motor functions; he just sat there and chattered away for a couple of minutes, then he was fine. Can you tell us what this is?
A: If your dog is a red cocker spaniel there is a disorder reported in them called multisytem neuronal degeneration. I have no clue how frequently it might occur but tremors and balance problems are seen with it. It occurs in young dogs, typically around 1 year of age.
If your cocker is not red it is possible that this problem may occur in other cockers since syndromes often affect more dogs than is initially supposed.
There are a lot of other possible causes of tremoring. Many of them are pretty obscure diseases and it is a good idea to consider asking your vet about referral to a neurologist if that is an option in your area. It is always a good idea to check a general blood panel and have a good exam done by your vet in case this is symptom of a general sytemic problem that your vet may recognize. Canine distemper, liver disease, kidney disease, some hormonal diseases and other systemic or organ system problems can lead to tremoring.
Mike Richards, DVM