Realistic thinking tells us, that they could be mini seizure even though it does not look like what we know a seizure to be in human terms.
All my life I have had Dachounds and they are prone to seizure activity even when well. The shaking that you speak of may very well be a petitte mal seizure. Also, at your pets age it could be a nerological issue. Over time, watch for sluggish movements and loss of tone, control in the limbs, this is, for sure a brain conection issue and needs treatment right away.
I look foward to helping you help yourself. If I do not have the answer to your question I will find someone who does. God Bless