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I would make sure that your radiator cap seals well like others have noted. You may need to add a thicker gasket or add a home cut gasket made from an old inner tube or similar material to your existing gasket. It should not leak from the cap regardless if hot or not.
Get a cheap dial meat thermometer and use it to check your coolant temperature so you know for sure what it is.
Did you happen to have the engine retarded while you were putting out the balloons?