The latest instalment of Anne-Sophie Mutter's massive Back to the Future series gave the impression that she might have bitten off more than she can chew. Her deeply personal retrospective of the 20th- century violin repertoire has passed from chamber music to concertos with surprisingly mixed results. The strengths and weaknesses of her playing, already apparent in two uneven but thrilling recitals, were, on this occasion, worryingly magnified.

With the LSO and Kurt Masur (on loan from the LPO), she essayed the Berg and Sibelius concertos back to back. The emotional trigger for Berg's masterpiece, written in 1935, was the death from polio of Manon Gropius, the 18-year-old daughter of Berg's friends Walter Gropius and Alma Mahler.

The opening cruelly exposed the flaw in Mutter's approach. She began with a bleached, vibrato-less tone, which she slowly weighted until the vibrato was full. The effect was aimed at giving the impression of gradually burgeoning life - but until she had strengthened the tone, her intonation was distressingly unsteady. Thereafter, she seemed oddly detached from the work until she reached the allegro, which depicts Manon's illness and death. Here Mutter briefly got into her stride, with those richocheting high notes sounding like shrieks of terrible pain. In the finale, however, a certain deliberation reigned once more. The concerto should leave you in bits. On this occasion I remained unmoved. Once past another vibrato-less, wonky opening, the Sibelius was much better, with a feistily dramatic first movement giving on to an Adagio which was immaculate.

Masur and the LSO were exemplary throughout, and the evening's true high point came, ironically, when Mutter was absent from the platform and they gave a performance of the Suite from Prokofiev's Romeo and Juliet which was, by turns, electrifying, deeply sexual and tremendously moving. The task that Mutter has set herself is daunting, but I was left wondering whether she has been over-ambitious.

Anne-Sophie Mutter plays the Barbican, London EC2 (020-7638 8891), tonight and Sunday