What's the pressure rating on the radiator cap? That one should probably be running a 4 lb. or maybe as much as a 7 lb. cap. The boiling point of coolant rises 3 degrees F. for every pound of pressure, so you could get up to 233 before it boils with a 7# cap. It looks like you're getting into slightly dangerous territory when the temp gets over 200, assuming the gauge is accurate.
Infra-red thermometers are available from places like Harbor Freight or Northern Tool, and they can be used to check the accuracy of the gauge without removing the radiator cap and exposing yourself to a potentially painful boil-over.
Jerry
