So, I'm not too sure what I'm looking for in terms of data, but using MIDI Monitor I've found no message that causes the clock to drop/audio to stop - but every time a the clock kicks back in, a sysex message is received - the description reads Universal Non Real Time.
So, according to the manual, a Universal Real Time message governs the clock...hmmmm. So, it doesn't matter which sysex mode the P6 is in, I'm still getting drops.
So, the question is - it this an Ableton deal or a DSI deal?
EDIT: Interestingly, these messages are transmitted every 4 minutes - even if the synth/Ableton is not playing.