Three works by three composers of three distinct temperaments and periods found the Juilliard String Quartet exercising one prevailing interpretative approach: sober. This worked well for two-thirds of its program Thursday night at the Library of Congress.

But the Juilliard customarily bats a thousand, which made the glaring lapses in Schumann's Quartet in F Major, Op. 41, No. 2, all the more puzzling. Aside from intonation problems, the performance was plodding. Instead of a free flow of melodic ideas, phrases labored under the overly cautious, almost protective delivery. Even the second-movement theme and variations lacked real sparkle.

Weightiness became David Diamond's String Quartet No. 10. This work, strongly influenced by Barto'k and Shostakovich, uses a repetitive syncopated motive to link the diverse, surprising modal strands in the three movements. Smooth ensemble legato bowing in the lento suddenly yielded to fitful outbursts. For the finale, a fugal theme passed from violins to cello to viola, grew to a feisty expanded dialogue, then played itself out as a variant of the opening motive returned.

At a time when Mozart can do no wrong, the Juilliard String Quartet proved that they could be equally weighty in its reading of his Quartet in D Major, K. 499. Mozart balanced this singular piece by placing the dramatic emphasis in the third-movement adagio. The quartet did likewise; a patient attack did full justice to the score. In exposing the composer's harmonic explorations, the Juilliard literally played into his hands.