As Isaac Stern eases out of mid-middle age into what promises to be the fullest phase of his musical life, he seems attracted to doing more, rather than less. Thus it was not enough, at last night's National Symphony pension fund concert, that he ended it with the grand proportions of the Brahms concerto. Before intermission there also was the Mendelssohn concerto.

No other violinist now playing has so fully come to terms with the broad spectrum of the repertory, and Stern apparently hungers to put his ideas to the test over and over -- as in the five programs he played here a year ago for his 60th birthday.

More than ever, he is an artist who strives more for song, and for getting exactly the right sound from his enoromous range, than for fireworks. He shows no interest in display unless it is essential to the music.

I, for one, would rather hear him play the Brahms now than hear anyone else. His basic ground plan seems set. Tempos and crucial passages were taken last night basically as they were a year ago. For instance, the soaring high song that ends the first movement was taken quite slowly both times, with ravishing sound (except for one missed note last night). There was one special feature last night: The very difficult up and down interval runs just before the return to the first movement's main theme were taken in almost pure legato; Lord knows how he did it.

Stern has cleansed the Mendelssohn of most of the unctuous ardor often ladled upon it. It is more classic, and quiet, than usual. The slow movement is faster than typical, but not rushed. With that piano that is like a whisper, he says more than could be expressed with a conspicuous throbbing vibrato.

The orchestra never sounds at its best at such special concerts, because they have to be squeezed in between the regular concerts prepared for the week, and there is little time for rehearsal. With Mstislav Rostropovich conducting, things were perfectly adequate. If the winds couldn't keep up with Stern in the opening measures of the Mendelssohn's last movment, they sounded first rate when it was repeated.