Just exactly what it is that makes the playing of the National Symphony Orchestra so erratic continues to elude this listener -- after all these years.

Friday before last we got a dramatic example, with a "Firebird" that was perfunctory following immediately upon a Strauss "Four Last Songs" with Jessye Norman that was molded with the greatest imagination and sensitivity. Last night, once again under principal guest conductor Rafael Fru hbeck de Burgos, the playing was of the highest order from the beginning to the end -- the finest thing this observer has heard from the National Symphony since its superb playing under former music director Antal Dorati in the fall.

The program was a succession of masterpieces -- which may be a factor in the quality. There was Wagner's ineffably tender "A Siegfried Idyll," the greatest birthday present any composer wrote for his wife, followed by his overture to "Die Meistersinger" -- arguably his most inspired. After intermission came Beethoven's "Eroica," as great a symphony as anyone ever wrote.

The performances were marvelously shaped. I have quibbled time and again with some of Fru hbeck's interpretations -- his Brahms in particular. But I couldn't complain about anything last night, except for some passing, and not very important, raggedness in three places in the "Eroica."

But it was the "Eroica" -- one of music's most formidable challenges -- that was the most impressive part of a fine evening. This was an "Eroica" that was decisive, that concentrated on the long line (an essential in the staggering "Marcia funebre") and that had enormous intensity. Fru hbeck was not just presenting an efficient run-through; he was going right to the heart of the music. Two tests: the great fugue in the "Marcia funebre," where Beethoven twisted the emotional noose as tightly as he could, hit you with enormous cumulative power; and the start of the concluding variations, a common trouble spot, took off like a thunderbolt.

Wagner's "Meistersinger" overture had a similar kind of electricity. The members of the orchestra were really playing together (nobody in recent years has questioned that the National Symphony has some awfully fine solo players).

Another feature: the quiet playing in "A Siegfried Idyll."

The program will be repeated tonight and Tuesday, and it's really worth a special effort to hear.