I would start with the thermostat. It is the easiest and least expensive.
Also if the temp gage is reading hot, if the temp sending unit is on the engine side of the thermostat and not the outlet hose side, the coolant in the block is hot. The coolant coming out of the radiator is not hot. I would have to presume that the coolant is being forced from the bottom of the radiator to the top. Which would be cool because it has been exposed to cooling air.