FIFTEEN YEARS AGO, Hair was a living, crying document of the agonies and ecstasies of the sixties. Today it's a period piece. While the original play had excitement, color, and passion, it's risky to try to clothe the show in modern dress and short hair without cheapening its spirit. The production now playing at the Institute of Politics uses a totally new plot as an excuse to string together the original songs. Although the show tries valiantly for three full hours, it never comes to life.

After the production of Antigone earlier this spring, there were high hopes that this version of Hair might be done with real power, raising ethical questions in a strikingly new way for a sometimes stodgy forum. But the very real issues that arise are quickly forgotten for the sake of a story line that evokes nothing so much as a rerun of the 1960s Monkees T.V. show.

While the show holds a few telling moments, time and again it wastes opportunities for real dramatic punch. The songs become tempting reminders of the power that might have been. "3500," a song about the agonies of Vietnam complete with pictures of the war in the back ground, provides a searing glance at the past, but the moment abruptly ends when one of the characters sarcastically remarks, "That was pleasant." And so the show returns to its banalities.

A scene in the closing moments, when one of the actors looks critically at the others' nostalgia for the ideals of the sixties, sums up the problem. Asking what it all really accomplished, he is neatly shunted aside by a quick and easy deus ex machina, the question never to be dealt with at all. The lone figure from the past--aged 21 in 1968, 41 in 1988 when the play is supposed to take place--serves only to frame the action and act as a convenient medium for the songs, like "Hair" and "Aquarius," that can't be neatly crammed into the modernized plot.

The preachy, heavy-handed script (a Pat Boone-style morality lesson in which it is other countries which cause problems, never the good ol' U.S. of A.) is doubly disappointing in that the show not only had the opportunity to evoke the spirit of the sixties, but the resources to do it with. Though not fully exploited here, the space of the ARCO Forum is a dynamic one to work with: Few other Harvard theatres offer the same staggered sight-lines or sense of openness and height. The technical capabilities add to the impact: The microphones (a godsend to this production in which some of the unaided voices do not reach the second row) and the overhead visual screen (although absurdly overused here) both create possibilities with which another show might experiment. It's encouraging to see the Institute used more like the original Roman forums, arenas for a community's expression in whatever form.

But more importantly, the production itself has several elements to commend it. The band is tight and together, giving a lot of energy to the snatches of the original Hair songs. John Gennari in particular not only delivers his lines with nice irony but gives an exciting performance on the drums. Several of the actors bring off their numbers with a flair and believability that belies the flatness of the characters they have to work with. It's unfortunate that so many of the classic songs have been so abbreviated, used only as links in the connect-the- songs script.

THE ACTING PERFORMANCES are uneven, although some fine efforts escape the limitations of the plot. The play's link with the sixties comes in the form of Greenie (Allen Gifford), the owner of the band's recording studio, who injects the struggles of two decades past, offering a suitably Messianic and inspirational figure to the jaded and cynical boys of the eighties. Brooks Whitehouse as Frank Mills, the leader of the band, and Dede Schmeiser as Donna Barona, his groupie girlfriend, on the other hand, fall a little short: Whitehouse a little stiff and stylized in his portrayal of an already implausible character, Schmeiser too broad and too brassy.

Other characters come off somewhat better. Freshman Merv Griffin gives a feeling of honesty to his role of Curtis Mann, the Janitor-turned-member of the band. Myrna Gellman (Sabrina Peck) has one of the livelier roles, and Peck provides the infectious enthusiasm and strong singing that the play must have to survive. Other characters: the British corporate heir of clipped accents and buried principles (Anthony Calnek) or the young committed activist, Barbara MacNeil (Sue Morris), carry off competent portrayals of paper-mache stereotypes.

Certainly no complaints can be made about the choreography, which really brings the numbers alive. Choreographer Peck has done an impressive job with the limited amount of space she had available. With actors and musicians who appear to have had little formal dance instruction, she has crafted several smooth and polished numbers, involving the entire cast as well as the dancers themselves. The exuberance of the dancing backs up the songs, a welcome constrast to the dryness of the action and the monotony of the set.

And although the language is sometimes painfully sophomoric, the message scattershot and blurred, one' has to admire Papas for attempting the overwhelming task of staging her own show. While the production carries flaws aplenty, it still makes for pleasant relief from the hackneyed productions of traditional shows that constitute the flotsam and jetsam of Harvard theatre. There's much naivete in Hair, but there's a lot of creativity too. Papas & Co., including the staff of the Institute, have to be admired for making a start toward establishing a tradition of orginial shows that try to convey a real message. It's only too bad that they could not have brought us more than snippets of the original Hair.