I have an 87 honda civic with 284K mile that is overheating. I have replaced the thermostat, water pump, all the belts and the radiator fan and the secondary fan. Any ideas? It only does it once the care is heated up and over 30mph. I am starting to think electrical.

Sounds like you could have a clogged radiator or you could have " air " in the system. Did you make sure to bleed the air from engine cooling system ? Some have a spout to bleed air from and others require that you set the car on an angle. ( weird but true )

Also is it an automatic ? How's the transmission doing ?