So, as per my original post, stop over-complicating your setup and trying to retain using the Montage as your tempo 'guide' (it sounds like you aren't using au Audio click, just a visual flashing button/indicator? It's really hard to understand from your descriptions which are somewhat vague and incomplete). I previous said that MIDI clock as a tempo sync method can be a little unreliable.
Just use ST as your Tempo source.
Whether that is a visual indicator, a click, whatever.
You are trying to combine 2 different 'solutions' (your old pre-ST method, and what you have created in incorporating ST just as a Setlist manager) and while you may have a better setlist management solution, you have a worse tempo management solution. You need to let go of the Montage playing a part in tempo management in your solution.