1997 Range Rover

As stated before, I think when you're mechanic did the head job, He may have forgotten to connect the temperature sending unit, or it got damaged during the head replacement, the reason I think that is, the temperature sending unit controls the fuel mixture at cold temperatures and hot temperature, as you described. It runs just fine when it is cold due to the fact that the computer is running a rich mixture, when the vehicle warms up the computer should lean out the fuel mixture According to the temperature, and because when the vehicle is hot it sounds like it's running rich, That might be caused by the temperature sending unit.