Because of the design of the engine bay in the XDs onwards, they tend to run hot, due to lack of air circulation. Just have a look at the engine bay of an XA, B or C and see the difference.
Because of the emission laws, the engines are also designed to run hot, as that assists in the dissipation of the gasses etc. The temp. gauge is configured accordingly by adding resistors in the right spots.
It is essential that the cooling system is maintained, and this generally means an annual flushing, with radiator renewal every 5 years or so, as the tranny cooler also sits inside the rad. and tends to dissolve over the years.
The main reason that the car tends to overheat in hot weather is the above not being done, and also look between the aircon. heat exchanger (rad) and the radiator, as this can also be filled with gunk.
A lot of EAs onwards end up beside the road with the binnet up in hot weather simply because the cooling system has not been maintained.