steveg,

It sounds like your cooling system is being overtaxed. If the fan runs constantly and will only hold 180&deg, it's just a matter of time until you overheat.

Your radiator is too small or plugged...or your fan setup is too small or installed wrong.

If all systems work properly, you should almost be able to select a operating temperature with a different temp thermostat.

I would investigate this problem before the first run up a steep hill on a hot day.

Stuart