Auto box dropping Revs

Hi Guys

I know this will end up being expensive- it always is !!

Anyway here goes -

I've noticed recently that whenever I move the gear selector from Park to Drive or Drive to Reverse the RPM drop really low - as though its about to stall, then bounce back up again, and become settled?

Any reason for this ?
and should i be concerned this could lead to something expensive ??

The car is a year 2000, P38, 4.6,

Thnaks as always

Check oil levels first. Maybe an oil change?

what deos she idle at to atart with, what does she drop to and what does she settle out at after?

Sounds like the stepper motor for the idle control wants cleaning.

Dont know where it is on a thor engine though. I've just done my Gems engine one and it made such a difference to the car.

wheres the stepper motor on the Gems engine?

It's under the throttle position sensor on the side of the metal casing with the throttle butterfly. held in by 2 x 8mm bolts

Thanks chaps for the replied so far

The precise figures are such - it idles nicely at 800, then when Drive or Reverse are selected from Park, it will drop to 400-500 then pick/ bounce back up again?

Seems totally bizarre,

Thankfully its hasn't stalled as yet but it seems as though it will any moment - imagine the embarrassment me with my blingy ( or as wife calls them chav) wheels and the car stalls !! -LOL