Devoting its summer music festival to works by Beethoven was not exactly a novel idea of the National Symphony Orchestra's. But ending the six-concert series Saturday night with the incidental music to Goethe's play "Egmont," under the expert direction of Christopher Keene, came just about as close to the unconventional as one can get with the works of so familiar a composer.

Like "Fidelio," this is another of Beethoven's strenuous efforts to grapple with writing music for the theater; it is a mode that did not come easily to him. In the 10 fragments of "Egmont," both his brilliance and his failings are plenteous. Few of his works, however, are more satisfying than the "Egmont" overture. It engulfs you like a torrent; it is tense as a wire spring. Before one idea has been digested, another comes shooting at you. With its strict structure and nervous tension, whether in the 5th Symphony or the "Egmont" overture, such writing is especially fitting for symphonic music, as in the overture. It doesn't so much flow as bolt.

But music of the theater is another thing, with its more discursive rhetoric and its wider time frame. Beethoven understood it but he never fully mastered it. He finally wrote a masterpiece in "Fidelio" almost in spite of himself, and in "Egmont" his handiwork is more erratic. The subject is, as Richard Freed observes in the National Symphony notes, "truly a work after his own heart" -- that is to say, defiance of tyranny to the point of death. The four entr'actes between scenes of the play, though, lack the riveting intensity that one expects of the composer; it is almost as if he feels that he must write rhapsodic connectives instead of going to the heart of the matter, which is exactly what he does in the overture, the concluding melodrama and "Siegsymphonie".

The National Symphony played as well as one could have asked; ensemble, balance and polish reached a level that would have been remarkable in the group a decade ago. Keene's interpretation fell just short of a high level of excitement, which is to say that it was quite good. Phyllis Bryn-Julson was her redoubtable self in the soprano solos. There was, though, one mistake, however minor. The narrator, Stefan Hurdalek, spoke in English except near the end when he changed briefly to German and was accompanied by the full orchestra. The reasoning must have been that the section is so operatic that German is essential. But the language should be uniform.

"Egmont" was preceded by the lyricism of Beethoven's violin concerto. The interpretation, with soloist Young Uck Kim, was straight, sober and rather dull. Fortune was not with him; he lost a string not long before the cadenza of the first movement. Instead of following the usual course and accepting the violin that concertmaster Miran Kojian offered, he went backstage and restrung his own.

One wondered where he got the cadenza in the last movement. It has the advantage of being shorter than the standard ones of Joachim and Kreisler. But there is little else good to be said for it.