First thoughts are that the stepper-motor (which controls the idle speed) is faulty. That would explain both the high idle at low engine temps and the stalling and stumbling as you roll to a stop.

You may have noticed that diesel drivers tend to idle their engines after driving, typically when finishing a journey, instead of switching off the engine immediately. This action may seem lazy or ...

The problem could have been the coolant, rather than the cap. A higher-pressure cap can help if it's a minor problem but, in any case, if you run into problems in the future there is an admission of a ...