Simple voltage regulator - problem with low temperature
Why are you using Darlington transistors in a low-voltage design? To begin with, the VBE of 1.4V is greater than your reference voltage &mdash; I'm surprised this circuit works at all! As it is, R32 has just millivolts across it, and R27 can have at most 10&times; that value across it (assuming Q12 is cut off completely).

For another thing, the minimum VCE of 1.5V is going to limit the available voltage to drive the MOSFET, which is operating in a sub-threshold region.

At a minimum, you need to select non-Darlington transistors for your differential amplifier. You don't need a huge amount of current gain in this circuit configuration anyway, but single transistors with gains of 200 or more are available. You may be happier switching to a design that uses a low-voltage rail-to-rail opamp.