Update #2 from OP:
I obtained another Montana 650t and tried it, with no difference in the performance.
I switched the Scala Rider G9 for another unit (my wife's paired G9) and no difference.
I contacted Cardo Systems via e-mail and I doubt they read my message as they replied with regards to Bluetooth functionality in their message, so I'm not impressed that they didn't bother to properly read my message and respond appropriately.
Contacted Cardo via telephone and explained my problem, but the only thing he could offer was to make sure the "click to link" was turned off. When I checked on this it was "off".
So for me, in my experience, the Scala Rider G9 is not compatible with the Garmin Montana 650t because of some type of interference with the Montana GPS satellite signal causing poor tracking and eventual loss of satellite reception with prolonged intervals before any sort of satellite reception is re-established. As such, I cannot recommend the Scala Rider G9 to be used with the Garmin Montana 6xx series GPS
Anyone using a Sena SMH10 successfully with a Garmin Montana 6xx series?