Garageband - Error code -39 - any clues?

I get the message 'Error code -39 was returned by the Core Audio driver' when I try and share a song with iTunes. It occurs during the mixdown phase, and from then on only a handful of tracks play!
I have 19 tracks in all - a mix of real and software instruments. I've locked all the tracks, but there is room for more optimization.
I'd appreciate any tips on how to solve this!