check the cooling fans for proper operation, they should turn on when the A/C is on, also check for dirt and debris in the cooling fins of both the radiator and heat exchanger out front. If the cooling fans work and you can see through the fins then the most likely culprit is the radiator, it is internally plugged and cannot handle more demand made by the A/C.
Some of the cooling fans in that era were bad about breaking the ring that holds the blades together. If the cooling fans are making a ticking noise it is time for a new one.
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