The Mostly Mozart Festival Orchestra plays together only in the summer, not all year. But its playing this week, under the batons of Raymond Leppard and Gerard Schwarz, can stand comparison with any full-time chamber orchestra.

It is understandably impressive with unfamiliar music, even with minimal rehearsal time. One example was Haydn's inventive 77th Symphony, with its earthy, country dance-flavored minuet, superbly conducted by Leppard on Wednesday night. Another was the slight, tuneful Cassation No. 1 in G, composed by the 11-year-old Mozart, which Schwarz brought to life last night. Here, the established orchestras have no advantage; the music is not in their repertoire.

But last night, Schwarz and the orchestra played two works that are ultra-familiar to audiences and orchestras everywhere -- Mozart's Fifth Violin Concerto and Beethoven's First Symphony -- and made them glow with the freshness and breathe with the vitality of brand-new music. Schwarz conducts with increased subtlety each time he comes to town, the orchestra's New York free-lance players are a formidable group. But there is more than that; this orchestra hasn't had time to become tired of what it is doing.

The soloists contributed enormously to the music's fresh impact. Flawless technique (particularly impressive in the first-movement cadenza) was only the surface in violinist Nadja Salerno-Sonnenberg's performance of the Mozart concerto. In the slow movement, she phrased with a warmth of feeling and richness of tone that called the name of Fritz Kreisler to mind, and her whole performance had a quality seldom heard in the work of nimble-fingered young virtuosi: elegance. It was a model of Mozart-playing, with a sense of style that many violinists never attain.

Pianist Grant Johannesen was not quite pedestrian but rather unadventurous in Beethoven's Op. 126 Bagatelles in the pre-concert recital. He played immaculately, focusing on the music's frequent contrasts, without looking much below its attractive surface. But in Chopin's youthful Variations on "La ci darem la mano," he was full of fire and poetry and technically dazzling.

The soloists played Mozart's Sonata in G, K. 301 simultaneously but not always together. The effect would have benefited from more rehearsal time to reach a consensus on phrasing and a better balance of sound.