Update:
Put it all back together and set the cold operation slide off.
Shutters opened this time exactly as I expected at the temperatures I measured (> 120 degrees). They did not before, and I think this was the real cause.
I think only time will tell now, I need a real outage, or for the family to head out for a while one day and I'll test it under a good solid load. I think it's going to be fine now, I really should not have run it with the cold setting on the air filter, I'm sure that was a contributing factor, as well as the shutters sticking being IMO the main cause.
Thanks for all the help!