I just listened to the whole song this time and the renderer starts dropping MIDI notes about 30 seconds in before dropping the rest of the MIDI tracks completely.

I had it split the tracks out individually and you can still definitely hear it.

I've never had a problem rendering before on this machine, so I am thinking it's something in BiaB 2011. 4Gib RAM, 2GHz. Dual-processor. One of the processors runs at 95% while rendering, but the other one stays relatively sleepy. Playing in real time puts no load on the system at all.

Do I need to freeze the MIDI track? It shouldn't be doing anything with those except playing what I tell it to through the soft synth... but maybe there is some lilt effect on the note timing? Why would it only show up during a render and not during normal playback?
--This is why I need to record the playback live as a workaround. I wanted to mail this version out today.