The snarling bark of the last movement of Beethoven's Opus 131 found the Juilliard String Quartet at its best last night at the Library of Congress. With its accustomed energy, the musicians tore into these chords with a conviction that matched Beethoven's own.

Throughout the rest of the program, however, there were some disturbing signs that the Juilliard is almost too virtuosic for its own, or the music's good.

The opening Reger Duo for two violins found Robert Mann and Earl Carlyss on different rhythmic wavelengths, Mann emphasizing solidity, which Carlyss, always on the front edge of the beat, seemed to strive primarily for flexibility.

The same sort of rhythmic ambiguity was evident in the Mozart Quartet, K. 464, which followed. So accustomed are the members of the Juillard to each other that their ensemble seems to be achieved almost as a byproduct of their playing side by side. When this sort of thing works, it is marvelous. When it doesn't, when the four are not thinking as one, it is unsettling, as it was last night.

The hyper-virtuosity surfaced in the early movements of the Beethoven as a disinclination on everyone's part to let a phrase be simple. Each small line was given exaggerated shape. The result sounded lumpy and overworked.