UNFAITHFULLY YOURS opens nationally. Directed by Howard Zieff. Written by Valerie Curtin, Barry Levinson and Robert Klane, based on Preston Sturges' 1948 film. Unlike Sturges' sophisticated black comedy, Zieff's remake lays on the slapstick and the screwball with Dudley Moore topining as a hot shot orchestra conductor who, after being erroneusly informed that his young wife is seeing a fellow conductor on the side, starts plotting her murder. For the most part, a manic but likeable diversion. With Nastassja Kinski, Armand Assante and Albert Brooks.