NOTHING DISTRESSES the sap-loving theater-goer more than watching the slow deterioration of this dowager of romantic musicals. Her originality is withered; her witticisms are expected; her style is frumpy and outdated by two decades. She survives on her past grandeur - barely enough to keep her flickering, let alone to satiate an audience. In short, My Fair Lady has become dowdy.

Director Greg Delawie has let all the lady's flabby flesh burst through her corset. He retains unaltered the play's weakest scenes, and even attempts to squeeze a size-14 chorus on the size-6 Dunster stage. The cast gamely bounces around but constantly risk stubbing toes on the furniture. When it isn't under orders to move, it sits blankly on the stage, obscurred from the view of everybody sitting farther back than the third row.

It seems that Delawie blithely envisioned an Alexandrian theatrical conquest without considering the limitations of his stage and cast. After all, even a threadbare musical like My Fair Lady can be mended with judicious shearing of cast and plot and modernizing of a few phrases of antiquated moralism. Delawie had innumerable versions to choose as models for his adaptation; he could even have set it in Cambridge and poked fun at Dorchester accents.

Instead, he sticks to the original as a toddler cleaves to its mother. When he ventures to emulate the style of another production, it is the oft-shown movies--itself remarkably faithful to the script. In the film version, hundreds of upper-crust stiffs assemble for the Ascot opening day races and stand at attention in overly starched collars without flexing a facial muscle.

The Dunster cast does the same. In fact, leads Selene Tompsett and Craig Hollander vaguely resemble Audrey Hepburn and Rex Harrison. The problem with trying to produce a show that mimicks a movie or even a Broadway production is obvious: students on a dining hall stage cannot hope to capture entirely the precision of professional performers. They end up looking silly.

Unconventional leads may have sparked the production, even if the supporting cast sagged. But both Henry Higgins (Hollander) and Eliza Doolittle (Tompsett) turn in standardized and mediocre performances. Tompsett's voice is low and well-modulated with a slight Southern softening, and though she tries to shrill, her slummy "Garn..." resonates with upper-class tonality. You can't make a sow's ear out of a silk purse. Only in scenes when Eliza is supposed to be furious with Higgins does Tompsett cast of her placid demeanor, and then she sizzles: her eyes splash cyanide when she seethes, "Just You Wait, 'enry 'iggins." She cannot sustain her fury, however; when Higgins dishes strawberry tarts to the bird and not to her, she looks only mildly miffed. Not even Tompsett's physical ploys, such as stalking off the stage or pushing her suitor, compensate for her lack of expression.

IN CONTRAST to Eliza's emotionalism, Higgins should be implacable, for a bohemian professor must remain oblivious to women. Hollander conveys that imperturbability, if somewhat blandly in the first few scenes. His solos, designed for enunciators rather than singers, display his rhetorical skills admirably. Unfortunately, the orchestra, even at low volume, drowns out about one-fourth of his and Tompsett's lyrics.

The one sparkle among the cast is Joshua Milton as Alfred P. Doolittle. Milton stares bug-eyed above the audience most of the time, but he cavorts and mugs with such ungainly enthusiasm and impious confidence that Santa Claus would envy him. Milton transforms the dustman into such a teddy bear you want to squeeze him and forget about his unsavory scruples.

Although the Dunster version of My Fair Lady shines during Milton's scenes and when Col. Picering (Marc Dolan--the one actor not cast in the movie's all-pervasive mold) calls the police to report Eliza's disappearance, the audience expects more. A production of one of the most popular musicals of all time--in which every song is a hit and the audience can practically recite favorite lines along with the actors--should not be picayune and imitative. The Dunster crew shows how worn a top-notch musical can become when it loses its youthful flair.