Subaru 2.2 will not idle smooth when cold. the idle will jump up and down from 400 to 3000 rpm when cold. once it warms up the idle will stay around 400rpm. what would cause this?

The Subaru engine could have a vacuum leak, bad head gasket, stuck choke, or several other things causing the problem. Any of these could give problems then ease up as the engine warms up.