I have a problem with my recently bought S Type 2.7SE diesel (2006) if you drive with a "feather" foot the car goes through gears smoothly if it is driven hard (or heavily laden) the car shakes badly between 30 and 50. The car has been back to the garage and had a second hand gearbox (from a 25000 mile car) but the fault is still present, the car is now to have an recon box fitted, has anybody had this fault or any ideas as to the cause, I am not certain that the box is at fault