1993 Nissan 300ZX Over heating problem

I changed the thermostat, the hoses, clutch fan. And it stills over heat. I can drive it but once I'm on the highway it will heat up but when the car is at a standstill it won't overheat... I also opened up the radiator cap when cold turned the car on to see if the coolant was flowing and it was. I'm just stuck now can you please help...