First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms you describe are possibly related to seizure, stress and/or a lesion causing alien hand syndrome.
Focal epilepsy can occur in the areas of language processing, temporal-parietal and inferior frontal cortex and the only manifestation may be speech arrest, inability to read, name objects, etc. These can occur without loss of conciousness, but people often get a strange feeling, or "seem different" during these seizures. Sleep deprivation can exacerbate epilepsy by lowering your seizure threshold, (so if you have a tendency to have seizures, you are more likely to have one under stress, such as sleep deprivation). An MRI can be normal in a patient with epilepsy. The test of choice would be an EEG (electroencephalogram or brain wave test). I would recommend a 2 hour sleep deprived EEG in your case, to start with. EEGs can be negative (falsely) if your spells do not happen while hooked up to the EEG, or if the seizure happens in a deep area of the brain that is not accessable to the scalp (where the electrodes are). The arm movements, both complex and simple can be caused by seizure, but since the symptoms do not always occur together in a "spell", you should also consider other possibilities as well.
Another possible cause would be alien hand syndrome. This occurs when there is a lesion (stroke, tumor, demyelination, trauma, etc.) that affects the Corpus callosum (white matter that connects the 2 sides of your brain), the frontal lobe (motor processing) and/or parietal lobe (sensory integration). The syndrome is characterized by the patients (usually Left) hand/arm does actions that do not appear to be under volitional control. Sometimes these are complex or simple movements. Occasionally the pateint also does not recognize the hand as their own. I do not know when you MRI was done, but I would recommend a repeat MRI with Contrast to rule out a tumor that may be invading your corpus callosum, etc. Alien hand syndrome would not explain your speech difficulties, but a lesion in the temoral lobe and invading the corpus callosum could cause both problems.
A third possibility is that myoclonic jerks are occuring in your left arm. Myoclonus is a brief jerk that can be caused by metabolic derangements, stress, and spinal cord lesions. These are also seen in some epileptic syndromes (such as Juvenile Myoclonic Epilepsy). For this I would recommend a CBC, CMP, Ca (with ionized) magnesium, TSH, ESR, CRP, ANA, ammonia. It may be that the sleep deprivation is causing your encephalopathy (altered awareness) and myoclonic jerks, but I would still recommend the above testing to evaluate for more serious causes.
I hope this has been helpful.
Please, have a full evaluation by a neurologist. They might repeat your MRI or add some contrast to it to visualize your brain better. You should also have an EEG, looking for possible seizures. This may be completely benign, but needs to be evaluated to make sure nothing like a seizure is going on. In the meantime, try to get better sleep hygiene as that can bring on these transient events.
Also have your doctor order carotid dopplers.
is probably not helping what is happening. The thing that struck me reading your post was that there is a type of seizure that can occur that affects only one arm, usually starting in the fingers and moving toward the shoulder - jacobson or jacobsonian is what I seem to recall from my college days. It commonly occurs after concussion type injuries when the brain literally hits the opposite side of the skull during an accident or injury.
The other thing that I would investigate further is your pineal region cyst, that gland regulates the melatonin your body produces and controls circadian rythyms, and can produce muscle weakness, tremors and swallowing probs amongst other things - did you have the blood work done to determine it was benign? I am going through that process now after a brain MRI for worsening tremors and muscle weakness beginning to affect swallowing. I have found the UCLA neuro website useful.
At the minimum I would say you need another neuro workup. Speech is controlled by a very specific part of the brain, as you are probably are aware. Sleep deprivation can cause speech probs (at 41 I refer to it as Mommy brain) I can find myself mid sentence without recalling what I started saying and I often find myself struggling to find the word I need or I am talking notice something while speaking and suddenly the word is in my sentence - but for me I can relate it to sleep deprivation, multi tasking and my level of concentration.
I hope you get it sorted out soon. You deserve the best of the best health wise working in your field. BTW, in my before kids life I was an occupational therapist :-)
Fiona