Cat Power, Shepherds Bush Empire, London

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Chan Marshall, aka Cat Power, is like a box of chocolates: you never know what you're going to get. Thus far in her career, the Atlanta-born singer-songwriter's performances have been more strawberry cream than truffle delight, known for her fits of crying, forgetfulness and stage fright. She's been responsible for some of the most terrible concerts of the past decade. But something has changed. The 36-year-old is now a bona-fide mainstream success, not just as a singer, but as an actress (in Wong Kar- Wai's My Blueberry Nights) and an improbable fashion muse to Karl Lagerfeld.

Has she lost her edge? Not at all. All the Chanel couture in the world wouldn't diminish her on-stage giddiness. From the start, she stalks the stage more like an excited 16-year-old than a seasoned professional, but it's mostly refreshing rather than grating. However, she's sleekly dressed in black and wearing some very Lagerfeld-esque biker gloves, and it's clear that the self-flagellating, plaid-shirted Marshall of yore, the one responsible for 1998's sparse, masterful Moon Pix, is dead. Marshall sings almost exclusively from her past two albums, 2006's The Greatest and Jukebox, her second covers album, which was released last week.

She opens with Billie Holiday's "Don't Explain", but its weary soulfulness is undercut by demented on-stage prancing. In 2006, backed by Teenie Hodges, one of Al Green's key sidemen, and the Memphis Rhythm Band, she delivered one of the most magnetic performances I've ever seen. Tonight, her backing band, the Dirty Delta Blues, is just as well chosen, with musicians from the Jon Spencer Blues Explosion and The Delta 72, and the Dirty Three's drummer, Jim White. But even they can't mask the fact that Marshall's singing is desperately unfocused. "Metal Heart" and "New York, New York" are tossed aside, rendered slight, lyrics swallowed.

She's still as charmingly kooky as ever: tap-dancing into the wings, pitching scrunched up set-lists into the crowd like a baseball player – and there's not a hint of nerves. Jimmy Buffett's break-up song "She's Got You" is delivered with equal sweetness and bile, and "Where is My Love" is heartbreaking, but The Greatest's best song, "Willie", is severely missing the soulful backing vocals and brass of the Memphis Rhythm Band. People start to leave, and barely anybody cheers for an encore. At her best, Marshall is the greatest singer you've ever seen and heard. At her worst, she's the worst. Could do better.