RidgeRunner45

My son's 1987 had a running hot problem but not to the same symptoms that you describe. In my case, it was the water pump. Replaced it and the problem was gone. Since you say it gets cooler with the AC "ON" means to me that the radiator cooling fan is kicking in with the AC "ON" and that is bringing down the overall cooling water temp. May have a faulty water temp switch, there is one located on the top of the radiator and should be a second on the bottom. Look for the electrical connector plug on the radiator to sensors that are screwed in the upper and lower radiator heads (engine side) and those should be the switches. The one on the bottom I believe controls the cooling fan so if you have to flip a coin and buy one, get the bottom one. You can check the switches by removing and getting your wife mad by putting them in her calaphon sauce pan and heating up in hot -boiling water. With a meter, check continuity Switch #1 (top) should come "ON" around just before the water boils, ~195 F and Switch #2 (bottom) should come "ON" with the water boiling. I am no certified auto mechanic so take my advice like you would from a friend with a beer in one hand and a greasy wrench in the other.