"Rick bounces back and forth between sickeningly optimistic and completely enraged."
(O comportamento de) Rick vai de um extremo ao outro, mudando/oscilando de absurdamente otimista a completamente encolerizado/enfurecido.
It doesn´t have to be abruptly (it may or may not), the emphasis is on the change of behavior and perhaps the frequency (that is, people would say so, when it happens frequently.)
Without context, I won´t state that Rick´s case is that of being sudden and unexpected. However it could be.
The definition of schizophrenic is something relating to the mental disorder characterized by hallucinations, delusions, disorganization and conflicting personalities.
An example of schizophrenic used as an adjective is the phrase "schizophrenic disorder" which means a disorder where a person changes personalities frequently.