Coolant cap on the radiator, or coolant cap on the over flow? There isn't much to a cooling system as far as things that are the reason for over heating. If the radiator is very old, you should replace it with a new one, as the fins get dirty/bent less air can flow through reducing the cooling performance. When/If you replace the radiator you should also replace the radiator cap. The cap has a spring pressure in it to pressurize the cooling system, this allows the cooling system to work better. No pressure, less cooling capacity, more over heaty. Your thermostat could be stuck closed, they generally fail open though so I wouldn't actually suspect this. And the radiator fans, you should know if these work. If it's over heating it should sound like a small tornado under the hood.
Coincidentally around 180-190k is when my RS' head gasket finally failed. It had been seeping for a long time but eventually dumped everything out.