Car at start up with cold motor revs it's guts out to past 2000rpm and then drops back to under 1000rpm after about 5 minutes of waiting. Whats causing this? Also I have noticed my car being AT. I have hardly any power pulling off a standing start while D is selected on the gear shift. The power just lags and car is struggling to get up past 100km/h in Drive. If I select S on the gear shift the power seems to return but even then is not at it's optimum. My cars gone from being a rocket to an old snail.
I would appreciate all your help.

Kind Regards
Hamish Caon.