Multiple Sclerosis can cause funny mixtures of symptoms. Have they actually proven the MS by tests? I'm just wondering if you have any other type of autoimmune disease, with antibodies against thyroid and/or parathyroid glands. You could ask Dr. Perlman about all this. I'd be surprised if she'd missed any of this, as she has a very good reputation, but even the best aren't perfect and things do get missed sometimes, so it's worth asking and getting HPTH and other autoimmune possibilities checked out.
As for other causes, are you on any medications which have any of these symptoms as side-effects?
Wilson's Disease has been known to cause HPTH, and can cause ataxia, dystonia, myoclonus and seizures, and many more. So do you have any copper rings around the outer edge of the iris, as per the picture on the Wikipedia link below? And has a Doctor done a blood test to check your Copper and Ceruloplasmin levels? Even if the copper rings are missing, and your MRI were negative, still get the blood tests done to make doubly sure. See:
"Myoclonus Dystonia" (DYT11) itself has occasionally been found to have Ataxia and even Seizures as symptoms, but this is somewhat rare.
DYT9 is also described as an episodic ataxia. See end of page 1 and start of page 2 here:
Dopa-Responsive Dystonia (DYT5, especially BH4 subtype) and SCA14 have both presented as Myoclonus-Dystonia with Ataxia and Seizures.
Finally, DYT18 is due to a problem with Glucose crossing the blood-brain barrier leading to low blood-sugar, causing a huge number of possible symptoms. A Lumbar Puncture and a blood test to compare the Glucose levels in the CSF with the blood can diagnose this, and lead to gene testing for GLUT1-DS2, which includes Dystonia 18, with or without Epilepsy.
Finally, some would say there is a link between HPTH and Parkinsonism, which might cause Dystonic like rigidity.
Hoping something here might sound familiar and be helpful.