[Gamling]

FYI, I discovered what was causing this... The Yamaha S80 synth sends MIDI clock messages when it is in its default config. I think this is because of the arpeggiator in the synth. This was effectively flooding the MIDI channel to the point where messages were getting lost.

Anyway, I disabled the SeqCtrl in MIDI setup and viola... MIDI works perfectly.

Just thought others might benefit from this information...

PS - MIDI Monitor was how I discovered what was going on behind the scenes. It's a great utility.