Dr. Jeckel And Ms. Hide 1990 Movie - Gender Transformations

Ashlyn Gere portrays the female version of a scientist who is transformed from man to woman by a potion he creates (not the intended effect). He switches between male and female form at random periods without control until quaffing a potion to stabilize his transformations -- purposely locking himself in female form when he is led to believe that the girl he is pursuing is a lesbian. Once he discovers that she isn't, he creates an antidote and is returned to his male self.