Sometimes in cold weather my engine will start but it won't idle after
I've used the accelerator. It started doing this last winter and the
problem has appeared again now it's got colder.

I believe it has to do with the idle speed stepper motor cutting off
it's airflow at the wrong time. When the engine is operating normally
this valve sounds like it is staying open when the accelerator is
pressed. However, when the engine is misbehaving this valve closes
when the accelarator is pressed, when the accelarator is released the
valve doesn't open in time and the engine stops.

I can get it working again by unplugging the stepper motor plug and
plugging it back in again. This only works when the engine is running.
I guess doing this resets the ECU in some way.

The car is a Peugeot 106 Rallye 98K miles with a Magneti Marreli 8P
ECU

I have measured the output of the coolant temperature sensor, the
speed sensor and the throttle position sensor and they seem fairy
sensible. I have recently changed the HT leads and the spark plugs,
and put some injector cleaner in. The vacuum pipes seem ok. I have
also sprayed some carb cleaner into one of the idle control air inlets
but I don't know if it did much.

Does anybody have any ideas what may be going on?