A shoteh is not a medical condition and is neither dependent on nor limited to specific illnesses, and I am not a mental health professional who can make specific diagnosis. The main point is that we are dealing with someone unable to be held responsible for his actions. The Gemorrah gives examples of behavior that would be typical of someone who is a shoteh: one who walks alone at night, one who sleeps in a cemetary and one who tears his clothing. Many schizophrenics and manic-depressives (bi-polar) would not express such extreme symptoms, while they may be likely in people suffering Down's syndrom or autism.
If the person alternates between being classified a shoteh and not, then when he is not in controll he is a shoteh and when he has controll of himself he is not.
Any determination l'maaseh must be made by a very qualified Rav in consultation with a mental health professional familiar with the patient in question.