2003 Mitsubishi Outlander - radiator seems to be not working

i love california summers but summertime means my radiator seems to quit on me. i always have overheating. but one curious thing is it seems to happen when the car is standing still (stoping at stop lights, just idling in a parking area, etc.)

i am an average car owner and i know how to fix minor problems. i plan to flush the radiator first. if that does not work, i am planning to replace the radiator or radiator fan or hoses, whichever needs to be replaced. i will bring my car to a mechanic or to the mitsubishi service area to check out the problem. but right now, i wanna know if you guys might know or experienced the same thing i am having right now and what you think about it. thanks!