Chevy 355, Holley systemax kit, Holley 750 carb. Advance curve kit, static timing about 16 degrees (per Holley instructions for cam). Dual electric fans, 180 T-stat, 3 core brass radiator. 16lb rad cap, 50 oz overflow tank.

After the engine gets up to 180, the thermostat opens and the temp stays there for about 10 minutes. Then the temp jumps (in about 15 seconds) to about 240. Within a minute it comes down to about 200 and stays there, sometimes going up to 210-215 depending on how hard I drive it. It does this if I am on highway speed, city, whatever. Always about 10 minutes into hot.

I am trying to figure what the cause is of the sudden heat? If the radiator was not efficient enough, why would it be ok for 10 minutes. Is there something in the engine heating up (intake maybe) causing a lack of proper cooling?

I will be installing a 2 core cross flow aluminum radiator, and a Taurus electric fan (4000 cfm for $50) soon, but I would still be curious on what opinions are on the cause.