I changed the heatsink compound and swapped the unit into production.
It is sitting at 29-31deg now in my basement and is stable.
I have left "Turbo" off, as I don't think it is needed or helpful on a passive cooled system.
FYI - anyone with the same unit. the igb7 and igb6 are reversed,
so it is igb0, 1, 2, 3, 4, 5, 7, 6