car sets idle at stop ;light starts running hot. water pump, cooling fan relay,radaitor fan switch, all replaced what could be the problem

sounds like the cooling fans are not working.check the temp switch. enough air is moving across the radiator and engine when you're rolling 55/75 mph or so,it's keeping it cool until you stop or get into stop and go traffic then no air moving much at all.

agree with answer # 1 have you change the Thermostat?