It would be handy to get the code that the ECU is coming up with. Willing to try plugging two connectors together under the dash to see if you can get the fault codes? It's not hard to do, "Rossco" posted a thread here on Perth WRX on how to do it (and I think there are pictures too!)

On the topic, I've had a similar idle problem pop up recently with a MY99/00 WRX, although not as bad as yours. With mine, when coming to a set of lights, or stop sign, the revs drop to about 500rpm and then slowly creep up to 750-800 rpm. Happens 50% of the time. No starting issues. The check engine light comes on when coasting in gear, down hill. The code for mine is 24 - Idle air control valve.

People who know Subarus well tell me this is a very common problem. It might be what you're experiencing too?

Try google but do be careful because some posts will suggest everything from airflow meter, to throttle position sensor, to the speedo not sending a speed signal to the ECU. Next thing, you're at the doctor because you think the red lump on your testicles is herpes.