Everything works PERFECT ..... BUT.... with a Performance (only this performance!!!!), anytime I play at the same time "Left Part with arpeggio and Right Part of Performance with Lead".... Click "reset" and go ... out of time.

Playing in time is still your responsibility. If you start an Arp off the beat - why would not it be out of time?
Maybe I’m not understanding what “only this Performance!!!” means, is it one you created or a Factory Performance?

Hopefully, you have resolved this issue... if not, let us know.
Certainly there are better ways to sync the band to Arpeggios than the method you are attempting. Let us know.

The general idea seems to match what's outlined for the Motif here: http://www.motifator.com/index.php/forum/viewthread/442575/#446629

When I saw "ARP SYNC: OFF" - I was left wondering if this was the Hold "Sync Off" - but I gather from the context that this should say "ARP SYNC QUANTIZE: OFF".

This still left me wondering what "reset" means. The "click 'reset' and go ..." statement. Not sure what the details are there and how to relate this to pushing buttons, piano keys, or the touchscreen on Montage. Or if this is somehow manipulating something that's MIDI (USB or DIN) connected to Montage.

With ARP Sync Quantize Off - you're less likely to get in sync than not.

This forum post covers that: https://www.yamahasynth.com/forum/syncing-arps-to-external-clock#reply-11308

CPU, DSP, or some failure of Montage to operate properly by any means are possibilities very last on a longer list of more reasonable explanations. Most likely a gap in understanding at the moment - soon to be bridged. It's difficult to "silver platter" as not all the details are here in order to reproduce the experience you're having.

Sometimes it's good to whittle down the steps to reproduce the same kind of perceived failure using simple changes to preset performances that others can follow (specific menu/button instructions).