Some moviegoers who missed playwright Peter Shaffer's absorbing Broadway smash "Equus" might dismiss director Sidney Lumet's film vesion as a kinky, if highly unlikely, psychodrama about an emotionally disturbed stableboy's erotic love for the horses, a sort of R-rated "My Friend Flicka." Yet, despite its faults - some sluggishness and a bit of overblown delivery on the part of Richard Burton, the boy's egomaniacal psychiatrist - "Equus" is worth seeing, if only for the haunting performance of Peter Firth, 24, as te stableboy.

Firth, hailed as a new sensation, co-starred in the play's London debut in 1973 and was considered a natural to make the film. For a year, it's said, Paramount held up the release of "Joseph Andrews," in which Firth plays a lighter, less dynamic role as Lady Booby's footman, timing its recent opening to capitalize on the antaicipated acclaim for the young British actor in United Artists' "Equus."

The film may not explode at the box office, but studio gamblers have hit the jackpot with Firth. His portrayal of Alan Strang, the illiterate, troubled youth who blinds six horses with a scythe, is so convincing that if he did that aimless shuffle, often associated with schizophrenic patients, into an emergency room, he would be admitted to the psychiatric ward straightaway.

Shaffer came across the idea behind "Equus" while driving around the British countryside, when he heard of such a bizarre crime but wasn't able to track down details. He felt compelled to give the incident some psychological rationale, he once explained, "to create a mental world in which the deed could be made comprehensible."

In the film, local magistrate Hesther Saloman (Eileen Atkins) refers Strang to a psychiatrist named Martin Dysart (Richard Burton), the Freudian sleuth who narrates, from behind his hospital desk, a sequence of sometimes jarring flashbacks into the boy's life and therapy. Strang's attraction for horses might unnerve equestrians, but the search for the riddle makes for a fascinating game of psychotherapeutic chess.

Dysart probes at Alan's memory and pays' his parents a visit. He learns, for example, that Alan's father, a repressed, prune-faced printer (Colin Blakely), has a penchant for skin flicks and once pulled his son off a horse. It was the boy's first ride, an exhilarating gallop down a beach with a stranger (a homosexual?). The fundamentalist mother (Joan Plowright) pumps Alan's empty head full of nightly Bible readings and the boy evolves a mystical, masochistic horse worship.

Horses, he seems to belive, suffered for man's sins. And he imagines he hears the voice of Equus (Latin for horse), a jealous god whose gaze he can never escape. Strang's first guilt-ridden sexual encounter with a woman fails in the hayloft of the barn, Equus' sacred temple, and he blames the horse-god. The bit of gruesome violence that ensues reportedly drew howls of protest from the SPCA until it was assured that the filmmakers had used wooden horses.

The film raises a host of questions. Why does a random event stir the cauldron of one psyche and not another? How does guilt-free ecstasy found in good old-time religion replace sex? Is it unfair for a psychiatrist to take away a patient's pain and replace it with sterile normality? Wonders Dysart, who leads a dull, lifeless existence and envies Strang's intense, if bizarre, passion.

"Equus," hardly a light outing at the movies, is only for the strong-willed looking for a moment of provocation.