Ive changed a lot of radiators and radiator fans on a few different brands of cars recently
A few had the problem you speak of and somtimes ive found the problem to be that the radiator itself isnt performing as well as it should be, through age and deterioration, though even if they look okay if they have been on for a long time (since new) or similar, they can scale up and block, and they become a lot less effective at low speeds/standing
if thats the case you can usually tell because you fan will cut in lots and almost straight away when standing still, unless ofcourse it is the thermostat which is always worth checking its cutting in and bringing the fan on at the right time so you know its working
