I also have an 03 Impreza TS (130,000 KM) and since about 2 weeks, the gaz doesn't go at idle when I hit the clutch to shift or to put on neutral. It does just the opposite. It goes up at 3000 to 4000 RPM and stays there for at least 15 to 20 seconds, so its really hard on the clutch not to mention safety issues.

I would like to know what causes that and what can be done to solve that situation (I guess this has something to do with the vacuum)?

Any help will be greatly appreciated.