''Spontaneously'' written and performed by the late Jeffrey Strouth from the back seat of an old car, American Fabulous is one unusual achievement. In telling his life story, the sardonic, witty, and abrasive Ohio-born monologuist a flamboyant cross between Clifton Webb, Tallulah Bankhead, and William S. Burroughs is uncommonly entertaining and just a little frightening. A

