2005 Nissan Tino Timing/Revs too high

Hi,
My used car is still under warranty, I had to send it back to have a few adjustments done, but it's come back worse that before!
The revs were sitting at approx 1200 rpm instead of 800 ish, it's now come back and on occasions is going up to almost 2500 rpm when pulling off and also changing up in gears. They have told me that they changed the Throttle Body Housing (which is what another Mechanic suggested could be the fault), and they said that it could take up to 200 miles for it to settle in and adjust? I find that bizarre, but is there any truth in that? It is racing at the moment and also using a lot of petrol.
I would be very grateful if you could offer some advice, as they are willing to pay for another garage to carry out the works now.
Many thanks
Sue