It's a problem that started with this version (2011). A song will start off well but will suddenly start dropping some or all of the notes. It can sound interesting when it is supposed to be playing back chords and instead just plays one note from each triad. Other times it just goes dead a few bars into a song. I haven't tried the beta but it doesn't mention this problem.

At least one old problem was fixed. One of the recent builds cured that wobbly timing problem on midi render, but the dropped note one is still there.