Darlington biasing design error in a commercial frequency counter oven control circuit?
Seems like it could be be marginal, especially since the Vbe drops on the Darlington pair decrease as the transistors heat.
I wonder if they designed it for some other op-amp and the JFET op-amp was substituted. I don't see any need for the JFET characteristics. That sort of thing tends to happen when the original designer moves on.