I've noticed at least twice now that if I'm in reverse and then I switch to Drive the car reves to around 1200 RPM and drops back down to 500 rpm within micro seconds. When that happens the car jumps a bit even when your foot on the break padle. Any idea why this is happening?

Also what it the normal RPM when the car is warm should be? I previous 7-series was around 700 RPM but the 545i is around 500 rpm sometime for few seconds it drops to 400 rpm which I think is pretty low. Any ideas what could be causing this? Thanks