In "She Stoops to Conquer," which opened at the Folger Theatre last night, the joke is on the upper class, in the person of a young suitor who has one set of manners for those he believes to be his social equals, and quite another for the poor slobs he deems beneath him.

Oliver Goldsmith's play is not, historically speaking, a Comedy of Manners, but it is a comedy about manners. And although Goldsmith had a serious point to make about the nature of theater (he was reacting to the sappy "drama of tears" that was popular at the time), today it's the jokes that survive.

The Folger production does not fail on the funny meter, but there is a sense of heaviness about it. It could be an overemphasis on interpretation at the expense of execution, a slight drag of pace where there should be briskness, a ritard where there should be a steady allegro. Some of it is no doubt attributable to the hint of discomfort that some of the performers have with Bary Allen Odum's delightfully elaborate costumes and wigs, a feeling that will surely pass. But a spark is needed to set this production alight.

Two of the main characters illustrate the problem. Mikel Lambert as Mrs. Hardcastle is a spark; outlandishly dressed and made up, she glories in the endearing silliness of her character. With a perfectly pretentious accent and grand extravagance of gesture, yet with every movement and syllable under control, she personifies the style that can make this type of comedy utterly delightful. The stage is alive when she is on it.

On the other hand, John Neville-Andrews' Mr. Hardcastle is unexpectedly tepid. This is the first role Neville-Andrews has undertaken since becoming the artistic producer of the theater, and he is by no means inept. But in creating a Hardcastle who is a contrast to the foppery around him, the man who disdains fashion and "loves every thing that's old," he has come up with a stiff and flavorless performance, encumbered by a strange shuffling walk that slows him down while failing to evoke age. He has his moments--notably a scene in which Hardcastle, playing Lord of the Manner to the hilt, lectures his strikingly incompetent servants, who all look like refugees from a production of "Marat/Sade," on their duties.

The plot is based on two devices: a practical joke and a romance. Young Marlow, a young swell en route to the Hardcastles' country residence to court Mr. Hardcastle's daughter Kate, is misdirected by her stepbrother, Tony Lumpkin. Tony tells the travel-weary Marlow and his friend Hastings, who have been too proud to ask for directions on their journey, that the Hardcastle home is in fact an inn. Marlow's presumptuous and "impudent" behavior toward Hardcastle stuns and horrifies his prospective father-in-law, whom the would-be suitor believes to be a humble innkeeper. The daughter, meanwhile, has discovered Marlow's problem: when among the "lower classes," he is confident and easy; when among his "own," he becomes tongue-tied and painfully shy. She dresses as a barmaid to woo him and succeeds.

Thomas Schall's Marlow is quite good, although he does not make enough of the young man's moments of stricken shyness. Lucinda Hitchcock Cone's Kate is bright and sassy, and Floyd King, who plays the minor role of the servant Diggory, proves once again that he is a most skilled and endearing clown. But Jim Beard's Tony Lumpkin is more reminiscent of the sullen brat Wackford Squeers in "Nicholas Nickleby" than the genial prankster the script seems to call for.

And the idea of having Diggory wave cutely to the audience after the curtain calls should be quickly canned.

SHE STOOPS TO CONQUER. By Oliver Goldsmith; directed by Davey Marlin-Jones; costumes by Bary Allen Odum; set by Lewis Folden; lighting by Hugh Lester; music by Tony Zito.

With Paul Norwood, Mikel Lambert, John Neville-Andrews, Lucinda Hitchcock Cone, Jim Beard, Chris Casady, Thomas Schall, John Wojda, Floyd King, Mario Arrambide, John Reese, David DiGiannantonio, Craif Paul Wroe, Kerry Waters and Gail Arias.

At the Folger through Feb. 27.