Subaru trys to die at stop and struggle to run

Throttle body, ISC and MAF clean? Are there any trouble codes? Even if the check engine light is not on you can have codes pending and stored. You might consider getting a live data scan done. All of this is dependent on good engine basics.

What do the plugs look like ?

In a twelve year old car, see if the engine's running lean or you have a vacuum leak. Or see if the air intake is leaking or the air filter housing isn't tight. It might be a defective idle air control valve/solenoid or defective mass airflow sensor. If you haven't had a good tuner-up in a long time, it might be an ignition problem, like old/fouled plugs.