I also had a problem with sulphur smell. Shortly after my check engine light had come on. Went and got the code run and the told me it was the O2 sensor. Apparently there was a recall on the 2000 Impreza 2.5RS for the O2 Sensor. The problem went away for about 20k miles but the check engine light came on today and the O2 sensor was the culprit. To bad it's no longer covered under my warranty any more.