“Equilibrium” is episode four of season three of Star Trek: Deep Space Nine.
Dax, after saying that none of her hosts had any musical talent, suddenly manages to successfully play a musical instrument. She then starts behaving erratically, snapping at people, before finally seeing hallucinations.
Doctor Bashir finds that one of her neurotransmitters is too low, and this could endanger the symbiont. He, along with Sisko, take Dax to the Trill homeworld, where the experts there will hopefully be able to figure out what is wrong with her.
Dax’s condition keeps deteriorating, though, and she may be having memories form decades ago coming out in her hallucinations. It seems that there is something in her past that is causing the hallucinations. Bashir and Sisko have to perform their own investigation because it looks like the Trill have covered up a traumatic event from Dax’s past.