We would need to know a little more about your cooling system before making an educated guess about what's going on. The Air Force term for that used to be a "SWAG" but that's not a forum friendly term- - - -the PC Patrol tends to get their knickers in a wad over that one!
Coolant boils slightly above 212 degrees F. at seal level unless it's pressurized. At 5,000 ft. altitude (Denver for instance) the boiling point drops to 205 or so. Every pound of pressure on a cooling system raises the boiling point 3 degrees, so the 7 lb. radiator cap that's pretty common on mid-1950's radiators takes the boiling point to 233 degrees. Modern cooling systems with high pressure radiators, heater cores, etc. can go to 15 PSI or more, with a boiling point above 250 degrees. Don't try this on a stovebolt, as the radiator core, or more likely the heater will explode.
A very clean stovebolt cooling system should rise to the thermostat opening point and stay there- - - -period! The temp might rise somewhat in traffic, or during prolonged periods of idling, or while pulling a heavy load up a long hill, etc. If you're experiencing temps above the stat setting, but less than boiling, something is restricting the coolant flow, the radiator or the engine water jacket is dirty, or a fan belt might be slipping, slowing down the water pump. It sounds like you've got a slightly dirty cooling system, or poor coolant circulation, or both.
Jerry
