1988 Buick Century Buck Keeps Over Heating Clue Runs About 5 Mins

First be sure the electric radiator fan is turning on. If it is, your mechanic can do a chemical test to see if a head gasket is leaking. One clue is you'll see air bubbles in the coolant reservoir but no steam to go with them, and that coolant will still be cold.

Run the heater on the highest temperature and fan speed. If that brings the engine temperature down, check the cooling fins on the radiator for signs of crumbling from corrosion. That will reduce its ability to give up its heat to the surrounding air.