AT HIS FREQUENT BEST, playwright Sam Shepard uses grisly explorations of family frictions and anxieties to roil up the audience in the way every good dramatist should. The Curse of the Starving Class, the last of a stark family-history trilogy, abounds in this desired therapeutic grittiness. Its characters' unpleasantness produce just the kind of irksome self-questioning and squirming that Aristotle prescribed 1500 years ago, the kind that makes going to the theater more than just entertainment. In Curse, only occasionally do Shepard, and the generally able troupe presenting it at the Loeb, stumble out of the realm of theatrical effectiveness into pure discomfort. What they leave is a production pockmarked with awkward moments and unpleasant breaks in tone.

From the first view of a rundown kitchen, complete with refrigerator and green plastic chairs, director R.J. Cutler and set designer Peter Sorger demonstrate a firm grasp of Shepard's hard realism. The Tates, a family of four, scrap endlessly about their unproductive farmstead and their dreary lives. Weston (Dean Norris), the alcoholic father, has just bashed in the front door after a night in the bars. His wife Ella (Nina Bernstein), who called the police to get rid of him, is having an affair with a slick town lawyer, and both husband and wife would like nothing more than to sell the house, gyp the spouse, and move out with the kids. The scene's immediacy--with the smell of frying ham drifting out to the audience as Ella starts breakfast--is gradually overwhelmed by an almost surrealistic sense of squalor.

That squalor, the play's major motive force, makes the production worth squirming for. Shepard's tools for inducing that Squirm aren't much subtler than the "starving class" metaphor of the title, which, despite numerous references in the dialogue, never surpasses the self-conscious (they're emotionally starving, you see). Emma (Molly White) the younger of the two gawky adolescents, is having her first period, as the mother constantly reminds father and brother to excuse her behavior. Wesley (Steven Gutwillig), her brother, urinates on a heap of Emma's painstakingly drawn posters. Shepard isn't one for the soft touch. His intention is obvious and a bit clumsy--to elicit from the audience the same plaintive reaction Emma has to Wesley: "What kind of family is this, anyway?"

THAT REACTION comes through loud and clear, helped by some strong actors and a director who sticks cautiously to a few basic note--squalor, loneliness, weired twistings of communication, and a creeping, gradually dominating mistrust. Cutler lets the cast carry Shepard's heavy messages, but he doesn't add much dramatic shape or thrust. Too ofen, the dialogue sags unbearably under the weight of its pregnant pauses; at other times, mostly in Act One, the actors lose track of their speeches meaning and say everything with the same flat air of significance.

White avoids this trap the best, enabling the consistently wacky pre-teen Emma to relate her personality so other characters--parents, for instance, Gutwillig as Wesley has a harder time. Less defined to start with, his character progresses a lot further through the plot's various unbelievabilities than his sister, and he and Bernstein, excellent by themselves, rarely convey the impression of mother and son. Most of the stiffness and bad timing evaporate once Norris as Weston comes onstage; the strongest personality of the four, he not only dominates the plot but brings most of the dramatic momentum with him.

During the more than two hours of trauma and suffering, spiked by the occasional one-liner, that momentum fortunately grows. By Act III even the wet-washcloth quality of the pauses can't distract us, though the pacing remains subtly off--the end of each scene, including the last, comes as a surprise letdown instead of a definitive period. In between, by way of atonement, Cutler has admirably showcased a parade of comedy bits, from the infamous live lamb to Keith Rogal's slimy portrayal of Corporate Evil as the interloping lawyer. Still, no amount of carbonation can lighten this load; Curse would weigh down the blithest spirit with distaste for these starving and unstarving misfits, compassion for their situation, and deep-rooted discomfort at their closeness to a twisted but familiar reality. And that, after all, is the point.