Test your thermostat in boiling water, does it open all the way (the spring compresses) at the thermostat temp. If not get a new one. Check the temp sensor wiring, or could be the sensor.
Make sure you have circulation in the cooling system, water pump flow and it's not actually overheating. Having a infrared temp gun is a good tool for old cars, then you can check/verify temp of of the whole motor, heads, upper/lower rad hoses etc.