[Gaz64]

John,

Both liquids are "coolant", if one is heating up, the other might go up also.

The thermostat dictates the minimum operating temp, the radiator dictates the maximum operating temp.

When moving at 30mph, the radiator should be capable of keeping the temp at the thermostat temperature.

If the engine begins to overheat only when stationary, you have two options:

Larger capacity radiator (BTUs), and/or larger capacity fan (CFM).