clio 93 overheating mystery?

hi
my car is over heating i have changed the thermostst, and thermo switch for the fan. i have bled my radiator and also added coolant, the garage are at loggerheads as to why the temp goes up too far, when its actually in the garage the fan works fine, but soon as i drive the car for a few miles then it over heats!!!!!the car is quite old, if the fan kicks in like it should when in the garage it isnt a blockage, the coolant is clean and free from sludge and oil. i do not lose any coolant,when the engine starts to overheat eg the needle goes into the red, the rad is still sold and the hose to the radiator is luke warm
can any one help me please??