Too many weak moments for the skilled Michelangelo String Quartet

The 90th-anniversary season of free concerts at the Library of Congress introduced a new ensemble to the Washington area on Saturday afternoon. The Michelangelo String Quartet, formed in 2002, had somehow eluded my notice up to this point. With its relatively new second violinist, Daniel Austrich, who replaced founding member Stephan Picard in 2012, the group's weaknesses temper enthusiasm for its strengths.