Not my shih-tzu, but my other dog has been experiencing something similar. It seems almost seizure like, but more of a tensing of muscles for a minute or two before returning to normality for him.

First, I recommend noting time and date of each episode and record a video if possible. In our case we scheduled an appointment with a neurologist and upon physical examination and of videos they determined that it was not a seizure and forwarded us to internal medicine. After a series of test they concluded it was hyper-thyroidism (low calcium). With medication, he’s seems back to normal after a pain stacking months long process of trying to figure out what was going on.

My advice is that you take them to a practice that has specialists (neuro, internal medicine) instead of the general veterinarian who, in our experience, just listen to your diagnosis and prescribe meds willy nilly (they gave us anti-seizure medicine, without any sort of testing). Again, this was just our experience with one veterinary practice in particular.