can't specify piano in General MIDI export

[Elwood54]

I had been exporting General MIDI with pianos and other instruments for months, but suddenly I'm unable to specify a simple piano. Out of a dozen or so instrument tracks with MIDI type notes that used to export just fine, I can specify the instrument for each of them except for the piano--which is strange since the piano is the default instrument. Instead, it sounds almost as though every note of what should be a piano is produced by a different instrument. I've specified Grand Piano in the patch, in the MIDI Events window, tried several exporting to different MIDI channels, but no luck. When I export an individual track, it plays on the QuickTime player as a piano, but I need the entire ensemble as a general MIDI file. It exports as AIFF perfectly. Any ideas as to what might cause this, or what I might try?

I'm using PTLE 7.3.cs1 (Mbox 2) on a dual G5 with OS X 10.4.9, Native Instruments Bandstand plugin for general MIDI playback and AIFF export. Any suggestions would be much appreciated.