1995 Land Rover Other Range Se 4 0 Put Gear Rpms Drop Almost Stall

When the vehicle is in Park it idles at about 900 rpm, when I put it in gear the rpms drop to about 200 and it either stalls or the rpms come back up and it idles fine. The rpms don't drop at any other time while driving or coming to stops. Just from Park to Reverse of Nutrual to Drive or Reverse.