1984 Toyota Supra heats up at high speeds?

You have known that the digital cluster is expensive to replace if you can even find one. A electronics expert might find a bad solder joint on the circuit board. 1984 is 26 years old. Almost always heating up at speed is caused by obstructed core tubes inside the radiator. A good radiator shop can take apart the metal tank type and rod out the core tubes. After the radiator then you may want new hoses heater throttle chamber and radiator hoses replaced that gets expensive. Good Luck