Gurjit

Hi , I am having a jaguar x type 2008 auto this and is my first car And bought it 2 months ago. But it is showing the gearbox fault when i start my car it drives perfectly but after some time it shows the gearbox fault all of the sudden and the car stuck at gear 3 rd but when i off the engine and start again it drives perfectly again further when i take it after 40 m/h it shows no fault at all. I have given it to the mechanic and he checks all the engine and could not found any problem at all. i checked online and people were having problem with there turbo engine but they havent drove the car for 2-3 weeks but i am using my car on daily basis can someone help me out what can be the problem.