How sure can we be that GPO4 - or, I suppose, the Aria Player - is fully compatible with Mac OS X 10.7 Lion? I've rather burnt my boats and upgraded my Mac and now, getting into one of my very occasional bouts of music making with Digital Performer and various virtual instruments, I'm getting some distinctly flaky behaviour from GPO. Some of the time it plays back just fine, but then I'll find a track - most recently a clarinet part - is playing an unwanted note in all the gaps between the wanted ones, producing a sort of faux-bagpipe effect which is, to say the least, undesirable! I can stop it by playing the offending 'drone' note on the MIDI keyboard, but it's far from ideal.
I've installed the latest version of the Aria Player (1.111 or whatever it is), but that predates Lion by some margin. I've also updated DP to 7.24 which MOTU claim is fully Lion-compatible. The only other virtual instrument I'm running at the moment is Modartt's Pianoteq, which doesn't seem to be causing any trouble.
I recently read some advice about using 'pregen' in DP wherever possible, for which the instrument's window must be closed and none of its MIDI tracks record-enabled, but this doesn’t seem to be helping.