Why do the idle increase?

I have a 2007 Palio 1.2.
I have had it for just over a month.I noticed after 2 weeks that the idle would sometimes stick at about 1100 or 1200.It normally idles at around 600 or so.When I took it back to the dealer and told them,they weren't sure- but said that they would look into it.At first they thought it was a sticky accelorator cable.Then they took the car for a drive themselves and saw that the idle in some places would go up to 2200.They said it was a dirty throttle body,so they took it apart and cleaned it and it seemed fine after that.
Now,for the last 3 days - it has been doing it again.I dont know what to do.I have now run out of ideas.
I have also noticed that it only starts to happen after we have had a few days of rain.Could this have caused anything ?
I have read quite a few replies regarding this,but i dont think it is the quality of petrol.I am not sure if it is the idle setting on the ECU,as it didnt happen for the first week and half or so that I had the car.
Can somebody PLEASE help me with this.
I have noticed that after i turn the engine off and leave it for about 10 minutes and restart it,it idles fine - but as soon as i drive off - the idle goes up again
PLEASE HELP !!!!

If the problem was solved on cleaning the throttle body. i think its time you get it changed. Another thing that could possibly cause such behavoiur is a bad lambda/oxygen sensor. I'm sure your problem will be solved on changing these two.