Range Rover L322 Gearbox problem

Hi all,
Newbie here and after some advice if possible please. Got a 2004 L322 a couple of weeks ago, and it drove fine, and still does tbh, but if you need to really put your foot down for anything (getting out in to fast moving traffic etc) it is almost as though the gearbox jumps out of gear. Is this likely to be that it just needs topping up with oil, or is it a more major issue ? Not very experienced with the L322. Love the car but a bit concerned I have been ripped off with this. Any help or advice appreciated. Thanks