Well i had the same problem with my vitz, i went to toyota several times and was given many temprory solutions, like,
- change the air filter
- tighten the battery clips
- change the plugs
- fill the high octane fuel
today on 22 june 2010 finally when i could not get any solution then toyota waltan motors simply told me that we dont have any adjustment solution for rpm of vitz, anyways i found a solution.
Car Care centre, ferozpur road, while coming from kalma chowk going towards general hospital, when crossing modeltown turn, just be in the service lane and it would be 500 meter on your left, i got the solution in just an hour and my car is absolutely fine now,
03004206121 is the contact number of engineer waleed owner of workshop.
i hope this will help to all , having problem with rpm of vitz