H.P. Lovecraft: The Shadow Out of Time (1975)

Based on H.P. Lovecraft's The Shadow Out of Time, this is the story of a professor who suffers a seizure which results in total amnesia. As he recuperates he is so different his family deserts him. He gathers knowledge of things outside his specialty for five years, then has another seizure. When he reawakens he is himself again, but now he has no memory of the five years between the attacks. He begins having bizarre dreams, and tries to find out what he was doing during the five years he can't remember. What he discovers, combined with the growing reality of his dreams, convinces him that something much darker than a mere personality shift was going on.