There's a coup d',etat afoot at Mount Zion Church in Harlem. The pastor, Sister Margaret, is losing her grip -- not only on her flock but on her teenage son. What's more, her long-lost lout of a husband, a consumptive jazz trombone player, has chosen this moment to reappear after 15 years' absence.

Such is the angle of "Amen Corner," a new musical adapted from a drama by James Baldwin. The world-premiere production at Ford's Theater -- which is bound, we are told, for Broadway -- shows promise here and there, especially in its rousing gospel tunes. But it suffers dearly on the cross of ragged acting and perpetual talk, and is liable to leave you yawning whenever you're not tapping your toes.

The two-act book, by Philip Rose and Peter Udell, dribbles out Baldwin's story like molasses from a bottle. Matters get moving with composer Garry Sherman and lyricist Udell's bouncy songs -- which make up in energy what they want in originality -- but inevitably they come to a halt in a surfeit of mawkish squawking.

This show has lines that cry out to be cut. A lot of them -- lazy clich,es such as "Maggie, you ain't hardly changed; you still the prettiest girl I ever laid eyes on" -- seem to suggest less about the characters' plight than they do about the authors'.

But then, what Rose & Co. find so difficult to dramatize in words suddenly comes alive in song. Sister Margaret and her husband Luke's moment of musical bickering, "You Ain't Gonna Pick Up Where You Left Off," boils with all the passion that their meaningful dialogues lack. And the show- stopping number, "In His Own Good Time," fairly bristles with behind-the-pulpit intrigue and smartly clarifies the various motives of Sister Margaret's rebellious congregants.

The performances range from appealingly salty -- such as Helena-Joyce Wright's sardonic Sister Boxer -- to downright embarrassing, such as Roger Robinson's Luke. Author Rose, who also directs, has indulged the cast to unseemly extremes, letting Robinson, for one, go all wet and weepy during one song and waggle and flick his tongue like an iguana during another.

Meanwhile, Luke's tubercular death, presumably a sad event, plays as one of the funniest scenes in the show, with Robinson suddenly slumping over cartoon-style on the sofa and opening his fist on what appears to be his trombone mouthpiece, which clatters noisily to the stage. "Rosebud" maybe?

As the heroine preacher, Rhetta Hughes is hampered by material that makes her hardly more sympathetic than your average tyrant, mindlessly shouting "Praise the Lord!" whenever anyone brooks her authority. But Hughes compounds the problem by playing it harsh and stiff. Any congregation, from all evidence, should count itself lucky to be rid of her.

The singing is fine on the whole, but the cast can't boast one outstanding gospel vocalist, which a show such as this would seem to demand. What little dancing there is, meanwhile, is safely unimaginative, and the sets of the church and Sister Margaret's apartment -- which are continually moved back and forth in front of each other like departure ramps at Dulles -- look as though they might have been thrown together for an out-of-town tryout. AMEN CORNER -- At Ford's through October 23.