With certain types of bad Head gaskets, the coolant temps stay normal when the car is operating in the street, and even when driven enthusiastically on the street.
However, when the engine is exposed to sustained high loads and RPMs, the cylinder nearest the weak spot in the gasket pumps air through the bad spot and into the coolant circuit, causing coolant temps to spike. When the engine cools off, the coolant may leak into the cylinder.
The coolant system should be pressure checked, and a leak down test performed on the cylinders.
Until these items are judged healthy, any work on the cooling system may be a band aid until the engine is again in a sustained high load state.