Strange idle issue

duster360 said
Just had a brainstorm. I wonder if my vacuum advance diaphragm is going bad. I run full manifold vacuum. If the diaphragm is bad that would drop my initial way down and the idle also.
But that wouldn’t explain the extreme lean condition on the AFR gauge

Possible, but like you said that doesn't explain the dying.
Especially like you said "Idled lower and lower until it died".
The vacuum issue would cause it to drop all at once.