Giant Sand's recent Cover Magazine (Thrill Jockey), a charmingly erratic all-covers album, is surely the slightest record ever produced by Howe Gelb's twisted Tucson roots-rock combo. But it's a logical step: Since the late 80s the band's guiding muse has been impulsiveness, if not quite improvisation; songs collapse into chaos, lyrics trigger free association, interludes erupt seat-of-the-pants style. It's made for many extremely dull, self-indulgent performances over the years, but when things do fall into place, as they did on 2000's Chore of Enchantment, the delicate balance of Americana songcraft, poetic unpredictability, and musical telepathy is all the more satisfying. On the new record, Gelb and his longtime rhythm section, Calexico's John Convertino and Joey Burns, treat the songs like jazz artists treat popular standards, using them to launch their own ideas and sometimes reinventing them in the process. Aided by nearly 20 guests, among them Kelly Hogan, Neko Case, and Polly Jean Harvey, they do Black Sabbath's "Iron Man" as a dark lounge gem, embroider Sonny & Cher's hit "The Beat Goes On" with Latin-esque piano, and (with Harvey) make X's nihilistic but spunky "Johnny Hit and Run Pauline" as creepily somnambulant as the rape victim of the title. The album does suggest that Giant Sand's unpredictability might be crystallizing into shtick; though Gelb injects his own arid wit into each song, his apparent boredom with established forms sometimes comes off as simply boredom. But in the past two years, the live show has been pretty consistently transcendent. The core trio is touring with trumpeter Noah Thomas, violinist Susan Voelz, bassist Laureline Prod'Homme, and guitarist Saholy Diavolana; onstage the players will come and go according to Gelb's whim. Sally Timms opens. Sunday, April 7, 9 PM, Abbey Pub, 3420 W. Grace; 773-478-4408.