The new production of "Camelot," which opened Tuesday at Lincoln Center, is one of those baffling juxtapositions of magnificent fatuousness, grace and leadeness that are a specialty of the American musical theater.

The house lights dim, the curtain rises and the senses are overwhelmed: with those glorious Lerner-and-Loewe songs; with a vast stage full of spectacular scenery, costumes and shields, swords, crowns, capes, pennants, spangles, castles and coats of arms all in dazzling royal colors; with an enthralling vision of medieval England a la Thomas Mallory and T. H. White.

And then, before we can bat an eye, wer are subjected to limp gags that are at once too new and too old; to plotting so abrupt and unsubtle it would be embarrassing in a comic strip; and to acting so wooden it should be thrown into a bonfire.

And in the middle of this hodgepodge stands the 54-year-old Richard Burton, a mighty confusion unto himself. Bruton reprtedly has bursitis, a painful condition of the joints, so the wonder may be that he is performing at all. For a performance that comes almost exclusively from one small section of the Burton anatomy -- his voice-box -- it is an extraordinary job.

Yet, there have been other headlines about Burton's medical history over the years, suggesting that he has not always fulfilled the actor's mandate to care for his body, his instrument. How is a poor theatergoer to know how much of Burton's current stiffness is due to a passing malady and how much to accumulated neglect? Well, he can't. The poor theatergoer knows only that he will be $10 to $30 poorer after seeing this most expensive production in Broadway history.

Alan Jay Lerner, author of the book and lyrics, apparently felt certain textual changes were in order for 1980. He has put the whole story into one long flashback, expanded the character of Merlin, eliminated that of Morgan le Fay, and made the young King Arthur no longer so young. It seems Arthur has already been on the throne for a number of years, doing nothing in in particular, before he marries Guenevere and begins to impose a new order on his realm. As long as he was rewriting, Lerner also might have concocted a little scene to establish, in Arthur's lengthy past, a history of lower-back trouble. That would have explained the peculiar way Burton is carrying himself these days.

Through much of the first act, he stands stiff and still, his hands limply at his sides, as if waiting for a tailer. Later, when he begins to move about the stage, the steps are so cautious that he seems to be in full (but invisible) armor.

Burton's early awkwardness may have something to do not only with his bursitis but with the fact that the age gap between actor and character is greater here. As the king grows older and his story grows more painful, Burton becomes steadily more assertive. Singing "How to Handle a Woman," he's relaxed, genuine and moving. "Camelot," the song John F. Kennedy left as a virtual anthem for his presidency, is frail and throaty the first time around, when the bashful Arthur in disguise woos his queen-to-be with a comic portrait of his kingdom as perfection. But at the final curtain, when Burton comes back to that song -- now as a recollection of a failed dream -- he takes thunderous, total command of the stage, the theater and the audience. 'it is as if he can will his ailment away for only a few minutes at a time. If so, he has certainly chosen his few minutes well.

Guenevere and Lancelot have not been aged, because neither Julie Andrews nor Robert Goulet was asked back for this production. The new Guenevere is Christine Ebersole, who took over the part on just a few days' notice in Toronto, leaving the role of Annie in "Oklahoma." She has a stunning voice and has here one of the most exhilarating sequences of song any musical has ever given any performer. But there is not much her acting (or any acting) could do with the nonsinging portion of her role. Once Guenevere has gone from baiting Lancelot to loving him in Act I, she simply gets lost in Act II.

As Lancelot, Richard Muenz makes a grand entrance on something that looks like a bramble bush on casters, and proceeds to sing "C'est moi, C'est moi" with a rich, house-filling voice and a nice feel for the comic side of it. mWhen he gets around to speaking, however, Muenz is hindered both by a musical-comedy French accent that he keeps abandoning just when we might be getting used to it, and by the strange way Lerner's book tosses off the whole romance of Lancelot and Guenevere.

When the lovers finally have their rendezvous in Act II, Lerner sums up the feeling between them with a few throwaway lines like, "Night after night I have thought of you here and have wished for it with all of my being" (that's Guenevere speaking), and "This agonizing torment! Day after day! Year after year!" (that's Lancelot). So much for love.

Elsewhere this pompous language is put into an incongruous mix with royal jesting that is anachronistic without being particularly fresh or funny. "Did you have a rough crossing?" Arthur asks Lancelot on his arrival from France. And King Pellinore, who keeps saying things like "Haven't got the foggiest," might have stepped straight out of a (lesser) P.G. Wodehouse novel, although Paxton Whitehead gives a divertingly dizzy performance.

Yet for all this silliness and stiffness, the score is so beautiful, so unified and so artfully used that when the three principals meet tragically at the end -- and Arthur recalls "this fleeting wisp of glory" they sustaned "for one brief shining moment" -- there is a tremendous poignant feeling of loss. Who cares then that what we have lost was never fully there on stage?