1984 Ford LTD Car STILL overheating?

Hello, I bought this car as a cheap form of transportation, and when I first bought it it was running great. Now that the weather is getting warmer out here, it has started overheating on relatively short trips (20-30 min. In stop and go traffic), and not able to be on the freeway for more than a couple miles before it overheats. It had a new thermostat and housing in '07, so we decided to put in an oil cooler. This has helped significantly for in city driving, but on the freeway it still overheats (although we can now go about 2-3 times farther than before). Do you think this is a cooling system issue? Or could it be a head/gasket problem? The next step we will take is getting the radiator flushed. I just don't want to keep putting money into the wrong area of the car.

Also, I should also mention that when the car is overheated and we let it cool down a bit, it doesn't idle right when we restart the car-it tries to die. But once we get going again it runs fine. Any help would be greatly appreciated.

Thank you!
-Dustin S