It could be the auxilliary air valve (sometimes called the air slide)
is 'tired' (faulty). Its what makes the engine idle faster when cold
(just started after long stop). As the coolant warms up, it heats up
the thermostat inside the device, which closes off the air way, and
reduces idle speed to the base idle speed.
In winter, the incoming cold air competes with the heating of the
coolant water acting on this valve, resulting in this valve remaining
open a little bit more than it maybe should. If the idle speed was
adjusted in this state, then the car will run OK when 'warm' in cold
In warmer weather, the incoming air is not so cold, so the valve will
close more. Since the idle speed was adjusted in cold weather, this
could result in the idle now being too slow, and the engine stalling.
The remedy may be to have a new auxilliary air valve fitted (this is a
bit of a pain, since it requires breaching the water circuit). Then
have the idle speed re-adjusted, preferably in warm weather, with the
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