1995 Chevy Caprice Need a computer?

The engine on the vehicle overheated -- very quickly - and the guage was way into the red zone. The car was towed to my home. A hose with a heater core was bad, and it was fixed. There was absolutely no water or antifreeze in the reservoir, so new antifreeze was added (over a gallon!). The car still overheats (within 10 minutes of the car running). The vehicle was towed to a mechanic, and now the mechanic is saying that I need a new "computer" for the vehicle. I'm a female, and I am very computer literate, but it seems to me that the car is not overheating because of the computer, but that the mechanic needs to hook up a diagnostic system to the computer (or he did and broke it!) And determine what is wrong with the vehicle. What is the answer? Does the computer going bad cause the overheating? Within the last year, I have had the water pump, thermostat, O2 censors, plugs, wires, and other major engine parts replaced. I realize it is an old car, and is not going to run like a new shiny vehicle, but do I have to replace every single part on the car every time something goes wrong? Help! [Please ask for more information if I have not provided enough.] I'll be willing to donate money for good advice, but now I am hurrying to get to the mechanic to see what the problem is.