Take a close look at this:
My 2000 Subaru Outback Sedan just starting acting up with the same problem so I’m goint to try that. This has to be related to a sensor providing bad data to the computer about airflow or air/fuel ratios. Somewhere there is a sensor that is only working correctly when the RPM climbs over idle… thus it’s working well enough for the computer to believe and react to the data from it. While I have it open, I’m also going to throw in a new air filter.